Document how to pass ENV vars to the user's shell

As this is a somewhat common question.

Docker-DCO-1.1-Signed-off-by: Sven Dowideit <SvenDowideit@docker.com> (github: SvenDowideit)

docs/sources/examples/running_ssh_service.Dockerfile

@@ -1,13 +1,17 @@
 # sshd
 #
-# VERSION               0.0.1
+# VERSION               0.0.2
 
-FROM    ubuntu:12.04
-MAINTAINER Thatcher R. Peskens "thatcher@dotcloud.com"
+FROM ubuntu:14.04
+MAINTAINER Sven Dowideit <SvenDowideit@docker.com>
 
 RUN apt-get update && apt-get install -y openssh-server
 RUN mkdir /var/run/sshd
-RUN echo 'root:screencast' |chpasswd
+RUN echo 'root:screencast' | chpasswd
+RUN sed -i 's/PermitRootLogin without-password/PermitRootLogin yes/' /etc/ssh/sshd_config
+
+ENV NOTVISIBLE "in users profile"
+RUN echo "export VISIBLE=now" >> /etc/profile
 
 EXPOSE 22
-CMD    ["/usr/sbin/sshd", "-D"]
+CMD ["/usr/sbin/sshd", "-D"]

docs/sources/examples/running_ssh_service.md

@@ -4,22 +4,27 @@ page_keywords: docker, example, package installation, networking
 
 # Dockerizing an SSH Daemon Service
 
+## Build an `eg_sshd` image
+
 The following `Dockerfile` sets up an SSHd service in a container that you
 can use to connect to and inspect other container's volumes, or to get
 quick access to a test container.
 
     # sshd
     #
-    # VERSION               0.0.1
+    # VERSION               0.0.2
 
-    FROM     ubuntu:14.04
-    MAINTAINER Thatcher R. Peskens "thatcher@dotcloud.com"
+    FROM ubuntu:14.04
+    MAINTAINER Sven Dowideit <SvenDowideit@docker.com>
 
     RUN apt-get update && apt-get install -y openssh-server
     RUN mkdir /var/run/sshd
     RUN echo 'root:screencast' | chpasswd
     RUN sed -i 's/PermitRootLogin without-password/PermitRootLogin yes/' /etc/ssh/sshd_config
 
+    ENV NOTVISIBLE "in users profile"
+    RUN echo "export VISIBLE=now" >> /etc/profile
+
     EXPOSE 22
     CMD ["/usr/sbin/sshd", "-D"]
 
@@ -27,6 +32,8 @@ Build the image using:
 
     $ sudo docker build -t eg_sshd .
 
+## Run a `test_sshd` container
+
 Then run it. You can then use `docker port` to find out what host port
 the container's port 22 is mapped to:
 
@@ -42,6 +49,22 @@ with `docker inspect`) or on port `49154` of the Docker daemon's host IP address
     # The password is ``screencast``.
     $$
 
+## Environment variables
+
+Using the `sshd` daemon to spawn shells makes it complicated to pass environment
+variables to the user's shell via the simple Docker mechanisms, as `sshd` scrubs
+the environment before it starts the shell.
+
+If you're setting values in the Dockerfile using `ENV`, you'll need to push them
+to a shell initialisation file like the `/etc/profile` example in the Dockerfile
+above.
+
+If you need to pass`docker run -e ENV=value` values, you will need to write a
+short script to do the same before you start `sshd -D` - and then replace the
+`CMD` with that script.
+
+## Clean up
+
 Finally, clean up after your test by stopping and removing the
 container, and then removing the image.