[docs] Update builder docs on last-modified times

Docker-DCO-1.1-Signed-off-by: Josh Hawn <josh.hawn@docker.com> (github: jlhawn)

docs/sources/articles/dockerfile_best-practices.md

@@ -103,7 +103,8 @@ a little more examination and explanation.
 being put into the image are examined. Specifically, a checksum is done
 of the file(s) and then that checksum is used during the cache lookup.
 If anything has changed in the file(s), including its metadata,
-then the cache is invalidated.
+then the cache is invalidated. The last-modified and last-accessed times of the
+file(s) are not considered in these checksums.
 
 * Aside from the `ADD` and `COPY` commands cache checking will not look at the
 files in the container to determine a cache match. For example, when processing

docs/sources/reference/builder.md

@@ -519,8 +519,8 @@ All new files and directories are created with a UID and GID of 0.
 In the case where `<src>` is a remote file URL, the destination will
 have permissions of 600. If the remote file being retrieved has an HTTP
 `Last-Modified` header, the timestamp from that header will be used
-to set the `mtime` on the destination file. Then, like any other file
-processed during an `ADD`, `mtime` will be included in the determination
+to set the `mtime` on the destination file. However, like any other file
+processed during an `ADD`, `mtime` will not be included in the determination
 of whether or not the file has changed and the cache should be updated.
 
 > **Note**: