# Get started with images In the [introduction](/introduction/understanding-docker/) we've discovered that Docker images are the basis of containers. In the [previous](/userguide/dockerizing/) [sections](/userguide/usingdocker/) we've used Docker images that already exist, for example the `ubuntu` image and the `training/webapp` image. We've also discovered that Docker stores downloaded images on the Docker host. If an image isn't already present on the host then it'll be downloaded from a registry: by default the [Docker Hub Registry](https://registry.hub.docker.com). In this section we're going to explore Docker images a bit more including: * Managing and working with images locally on your Docker host. * Creating basic images. * Uploading images to [Docker Hub Registry](https://registry.hub.docker.com). ## Listing images on the host Let's start with listing the images we have locally on our host. You can do this using the `docker images` command like so: $ docker images REPOSITORY TAG IMAGE ID CREATED VIRTUAL SIZE training/webapp latest fc77f57ad303 3 weeks ago 280.5 MB ubuntu 13.10 5e019ab7bf6d 4 weeks ago 180 MB ubuntu saucy 5e019ab7bf6d 4 weeks ago 180 MB ubuntu 12.04 74fe38d11401 4 weeks ago 209.6 MB ubuntu precise 74fe38d11401 4 weeks ago 209.6 MB ubuntu 12.10 a7cf8ae4e998 4 weeks ago 171.3 MB ubuntu quantal a7cf8ae4e998 4 weeks ago 171.3 MB ubuntu 14.04 99ec81b80c55 4 weeks ago 266 MB ubuntu latest 99ec81b80c55 4 weeks ago 266 MB ubuntu trusty 99ec81b80c55 4 weeks ago 266 MB ubuntu 13.04 316b678ddf48 4 weeks ago 169.4 MB ubuntu raring 316b678ddf48 4 weeks ago 169.4 MB ubuntu 10.04 3db9c44f4520 4 weeks ago 183 MB ubuntu lucid 3db9c44f4520 4 weeks ago 183 MB We can see the images we've previously used in our [user guide](/userguide/). Each has been downloaded from [Docker Hub](https://hub.docker.com) when we launched a container using that image. We can see three crucial pieces of information about our images in the listing. * What repository they came from, for example `ubuntu`. * The tags for each image, for example `14.04`. * The image ID of each image. > **Note:** > Previously, the `docker images` command supported the `--tree` and `--dot` > arguments, which displayed different visualizations of the image data. Docker > core removed this functionality in the 1.7 version. If you liked this > functionality, you can still find it in > [the third-party dockviz tool](https://github.com/justone/dockviz). A repository potentially holds multiple variants of an image. In the case of our `ubuntu` image we can see multiple variants covering Ubuntu 10.04, 12.04, 12.10, 13.04, 13.10 and 14.04. Each variant is identified by a tag and you can refer to a tagged image like so: ubuntu:14.04 So when we run a container we refer to a tagged image like so: $ docker run -t -i ubuntu:14.04 /bin/bash If instead we wanted to run an Ubuntu 12.04 image we'd use: $ docker run -t -i ubuntu:12.04 /bin/bash If you don't specify a variant, for example you just use `ubuntu`, then Docker will default to using the `ubuntu:latest` image. > **Tip:** > We recommend you always use a specific tagged image, for example > `ubuntu:12.04`. That way you always know exactly what variant of an image is > being used. ## Getting a new image So how do we get new images? Well Docker will automatically download any image we use that isn't already present on the Docker host. But this can potentially add some time to the launch of a container. If we want to pre-load an image we can download it using the `docker pull` command. Let's say we'd like to download the `centos` image. $ docker pull centos Pulling repository centos b7de3133ff98: Pulling dependent layers 5cc9e91966f7: Pulling fs layer 511136ea3c5a: Download complete ef52fb1fe610: Download complete . . . Status: Downloaded newer image for centos We can see that each layer of the image has been pulled down and now we can run a container from this image and we won't have to wait to download the image. $ docker run -t -i centos /bin/bash bash-4.1# ## Finding images One of the features of Docker is that a lot of people have created Docker images for a variety of purposes. Many of these have been uploaded to [Docker Hub](https://hub.docker.com). We can search these images on the [Docker Hub](https://hub.docker.com) website.  We can also search for images on the command line using the `docker search` command. Let's say our team wants an image with Ruby and Sinatra installed on which to do our web application development. We can search for a suitable image by using the `docker search` command to find all the images that contain the term `sinatra`. $ docker search sinatra NAME DESCRIPTION STARS OFFICIAL AUTOMATED training/sinatra Sinatra training image 0 [OK] marceldegraaf/sinatra Sinatra test app 0 mattwarren/docker-sinatra-demo 0 [OK] luisbebop/docker-sinatra-hello-world 0 [OK] bmorearty/handson-sinatra handson-ruby + Sinatra for Hands on with D... 0 subwiz/sinatra 0 bmorearty/sinatra 0 . . . We can see we've returned a lot of images that use the term `sinatra`. We've returned a list of image names, descriptions, Stars (which measure the social popularity of images - if a user likes an image then they can "star" it), and the Official and Automated build statuses. [Official Repositories](/docker-hub/official_repos) are a carefully curated set of Docker repositories supported by Docker, Inc. Automated repositories are [Automated Builds](/userguide/dockerrepos/#automated-builds) that allow you to validate the source and content of an image. We've reviewed the images available to use and we decided to use the `training/sinatra` image. So far we've seen two types of images repositories, images like `ubuntu`, which are called base or root images. These base images are provided by Docker Inc and are built, validated and supported. These can be identified by their single word names. We've also seen user images, for example the `training/sinatra` image we've chosen. A user image belongs to a member of the Docker community and is built and maintained by them. You can identify user images as they are always prefixed with the user name, here `training`, of the user that created them. ## Pulling our image We've identified a suitable image, `training/sinatra`, and now we can download it using the `docker pull` command. $ docker pull training/sinatra The team can now use this image by running their own containers. $ docker run -t -i training/sinatra /bin/bash root@a8cb6ce02d85:/# ## Creating our own images The team has found the `training/sinatra` image pretty useful but it's not quite what they need and we need to make some changes to it. There are two ways we can update and create images. 1. We can update a container created from an image and commit the results to an image. 2. We can use a `Dockerfile` to specify instructions to create an image. ### Updating and committing an image To update an image we first need to create a container from the image we'd like to update. $ docker run -t -i training/sinatra /bin/bash root@0b2616b0e5a8:/# > **Note:** > Take note of the container ID that has been created, `0b2616b0e5a8`, as we'll > need it in a moment. Inside our running container let's add the `json` gem. root@0b2616b0e5a8:/# gem install json Once this has completed let's exit our container using the `exit` command. Now we have a container with the change we want to make. We can then commit a copy of this container to an image using the `docker commit` command. $ docker commit -m "Added json gem" -a "Kate Smith" \ 0b2616b0e5a8 ouruser/sinatra:v2 4f177bd27a9ff0f6dc2a830403925b5360bfe0b93d476f7fc3231110e7f71b1c Here we've used the `docker commit` command. We've specified two flags: `-m` and `-a`. The `-m` flag allows us to specify a commit message, much like you would with a commit on a version control system. The `-a` flag allows us to specify an author for our update. We've also specified the container we want to create this new image from, `0b2616b0e5a8` (the ID we recorded earlier) and we've specified a target for the image: ouruser/sinatra:v2 Let's break this target down. It consists of a new user, `ouruser`, that we're writing this image to. We've also specified the name of the image, here we're keeping the original image name `sinatra`. Finally we're specifying a tag for the image: `v2`. We can then look at our new `ouruser/sinatra` image using the `docker images` command. $ docker images REPOSITORY TAG IMAGE ID CREATED VIRTUAL SIZE training/sinatra latest 5bc342fa0b91 10 hours ago 446.7 MB ouruser/sinatra v2 3c59e02ddd1a 10 hours ago 446.7 MB ouruser/sinatra latest 5db5f8471261 10 hours ago 446.7 MB To use our new image to create a container we can then: $ docker run -t -i ouruser/sinatra:v2 /bin/bash root@78e82f680994:/# ### Building an image from a `Dockerfile` Using the `docker commit` command is a pretty simple way of extending an image but it's a bit cumbersome and it's not easy to share a development process for images amongst a team. Instead we can use a new command, `docker build`, to build new images from scratch. To do this we create a `Dockerfile` that contains a set of instructions that tell Docker how to build our image. Let's create a directory and a `Dockerfile` first. $ mkdir sinatra $ cd sinatra $ touch Dockerfile If you are using Docker Machine on Windows, you may access your host directory by `cd` to `/c/Users/your_user_name`. Each instruction creates a new layer of the image. Let's look at a simple example now for building our own Sinatra image for our development team. # This is a comment FROM ubuntu:14.04 MAINTAINER Kate Smith RUN apt-get update && apt-get install -y ruby ruby-dev RUN gem install sinatra Let's look at what our `Dockerfile` does. Each instruction prefixes a statement and is capitalized. INSTRUCTION statement > **Note:** > We use `#` to indicate a comment The first instruction `FROM` tells Docker what the source of our image is, in this case we're basing our new image on an Ubuntu 14.04 image. Next we use the `MAINTAINER` instruction to specify who maintains our new image. Lastly, we've specified two `RUN` instructions. A `RUN` instruction executes a command inside the image, for example installing a package. Here we're updating our APT cache, installing Ruby and RubyGems and then installing the Sinatra gem. > **Note:** > There are [a lot more instructions available to us in a Dockerfile](/reference/builder). Now let's take our `Dockerfile` and use the `docker build` command to build an image. $ docker build -t ouruser/sinatra:v2 . Sending build context to Docker daemon 2.048 kB Sending build context to Docker daemon Step 1 : FROM ubuntu:14.04 ---> e54ca5efa2e9 Step 2 : MAINTAINER Kate Smith ---> Using cache ---> 851baf55332b Step 3 : RUN apt-get update && apt-get install -y ruby ruby-dev ---> Running in 3a2558904e9b Selecting previously unselected package libasan0:amd64. (Reading database ... 11518 files and directories currently installed.) Preparing to unpack .../libasan0_4.8.2-19ubuntu1_amd64.deb ... Unpacking libasan0:amd64 (4.8.2-19ubuntu1) ... 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Running hooks in /etc/ca-certificates/update.d....done. ---> c55c31703134 Removing intermediate container 3a2558904e9b Step 4 : RUN gem install sinatra ---> Running in 6b81cb6313e5 unable to convert "\xC3" to UTF-8 in conversion from ASCII-8BIT to UTF-8 to US-ASCII for README.rdoc, skipping unable to convert "\xC3" to UTF-8 in conversion from ASCII-8BIT to UTF-8 to US-ASCII for README.rdoc, skipping Successfully installed rack-1.5.2 Successfully installed tilt-1.4.1 Successfully installed rack-protection-1.5.3 Successfully installed sinatra-1.4.5 4 gems installed Installing ri documentation for rack-1.5.2... Installing ri documentation for tilt-1.4.1... Installing ri documentation for rack-protection-1.5.3... Installing ri documentation for sinatra-1.4.5... Installing RDoc documentation for rack-1.5.2... Installing RDoc documentation for tilt-1.4.1... Installing RDoc documentation for rack-protection-1.5.3... Installing RDoc documentation for sinatra-1.4.5... ---> 97feabe5d2ed Removing intermediate container 6b81cb6313e5 Successfully built 97feabe5d2ed We've specified our `docker build` command and used the `-t` flag to identify our new image as belonging to the user `ouruser`, the repository name `sinatra` and given it the tag `v2`. We've also specified the location of our `Dockerfile` using the `.` to indicate a `Dockerfile` in the current directory. > **Note:** > You can also specify a path to a `Dockerfile`. Now we can see the build process at work. The first thing Docker does is upload the build context: basically the contents of the directory you're building in. This is done because the Docker daemon does the actual build of the image and it needs the local context to do it. Next we can see each instruction in the `Dockerfile` being executed step-by-step. We can see that each step creates a new container, runs the instruction inside that container and then commits that change - just like the `docker commit` work flow we saw earlier. When all the instructions have executed we're left with the `97feabe5d2ed` image (also helpfully tagged as `ouruser/sinatra:v2`) and all intermediate containers will get removed to clean things up. > **Note:** > An image can't have more than 127 layers regardless of the storage driver. > This limitation is set globally to encourage optimization of the overall > size of images. We can then create a container from our new image. $ docker run -t -i ouruser/sinatra:v2 /bin/bash root@8196968dac35:/# > **Note:** > This is just a brief introduction to creating images. We've > skipped a whole bunch of other instructions that you can use. We'll see more of > those instructions in later sections of the Guide or you can refer to the > [`Dockerfile`](/reference/builder/) reference for a > detailed description and examples of every instruction. > To help you write a clear, readable, maintainable `Dockerfile`, we've also > written a [`Dockerfile` Best Practices guide](/articles/dockerfile_best-practices). ### More To learn more, check out the [Dockerfile tutorial](/userguide/level1). ## Setting tags on an image You can also add a tag to an existing image after you commit or build it. We can do this using the `docker tag` command. Let's add a new tag to our `ouruser/sinatra` image. $ docker tag 5db5f8471261 ouruser/sinatra:devel The `docker tag` command takes the ID of the image, here `5db5f8471261`, and our user name, the repository name and the new tag. Let's see our new tag using the `docker images` command. $ docker images ouruser/sinatra REPOSITORY TAG IMAGE ID CREATED VIRTUAL SIZE ouruser/sinatra latest 5db5f8471261 11 hours ago 446.7 MB ouruser/sinatra devel 5db5f8471261 11 hours ago 446.7 MB ouruser/sinatra v2 5db5f8471261 11 hours ago 446.7 MB ## Image Digests Images that use the v2 or later format have a content-addressable identifier called a `digest`. As long as the input used to generate the image is unchanged, the digest value is predictable. To list image digest values, use the `--digests` flag: $ docker images --digests | head REPOSITORY TAG DIGEST IMAGE ID CREATED VIRTUAL SIZE ouruser/sinatra latest sha256:cbbf2f9a99b47fc460d422812b6a5adff7dfee951d8fa2e4a98caa0382cfbdbf 5db5f8471261 11 hours ago 446.7 MB When pushing or pulling to a 2.0 registry, the `push` or `pull` command output includes the image digest. You can `pull` using a digest value. $ docker pull ouruser/sinatra@cbbf2f9a99b47fc460d422812b6a5adff7dfee951d8fa2e4a98caa0382cfbdbf You can also reference by digest in `create`, `run`, and `rmi` commands, as well as the `FROM` image reference in a Dockerfile. ## Push an image to Docker Hub Once you've built or created a new image you can push it to [Docker Hub](https://hub.docker.com) using the `docker push` command. This allows you to share it with others, either publicly, or push it into [a private repository](https://registry.hub.docker.com/plans/). $ docker push ouruser/sinatra The push refers to a repository [ouruser/sinatra] (len: 1) Sending image list Pushing repository ouruser/sinatra (3 tags) . . . ## Remove an image from the host You can also remove images on your Docker host in a way [similar to containers]( /userguide/usingdocker) using the `docker rmi` command. Let's delete the `training/sinatra` image as we don't need it anymore. $ docker rmi training/sinatra Untagged: training/sinatra:latest Deleted: 5bc342fa0b91cabf65246837015197eecfa24b2213ed6a51a8974ae250fedd8d Deleted: ed0fffdcdae5eb2c3a55549857a8be7fc8bc4241fb19ad714364cbfd7a56b22f Deleted: 5c58979d73ae448df5af1d8142436d81116187a7633082650549c52c3a2418f0 > **Note:** In order to remove an image from the host, please make sure > that there are no containers actively based on it. # Next steps Until now we've seen how to build individual applications inside Docker containers. Now learn how to build whole application stacks with Docker by linking together multiple Docker containers. Test your Dockerfile knowledge with the [Dockerfile tutorial](/userguide/level1). Go to [Linking Containers Together](/userguide/dockerlinks).