Filter containers by status.

A continuation of #7616.
Closes #7616.
Adds `docker ps --filter=status=(restarting|running|paused|exited)` option.

Docker-DCO-1.1-Signed-off-by: Jessica Frazelle <jess@docker.com> (github: jfrazelle)
Make second clause a separate sentence, add comma, fix tense and article: "...below the root directory. For example, the file `/somedir/subdir/temporary.txt` would get ignored in this case."