Add container sub-commands to the build command

[svrana]

Especially if you're using more than one docker file, it would be nice to have an easy way to build a particular container instead of rebuilding them all.

i.e., Say project foo relies on container nginx which i'd like to modify. It would be nice to do this:

dev foo build nginx

What i do now is run DEV_CONFIG=debug dev build foo and then ctl-c as soon as I see the build start and take a look at the logs to see the exact docker-compose invocation. Then I copy that and add 'nginx' at the end of it. After building i just run dev foo up to get all the containers to restart. It's not horrible, but could be made easier by adding all the container dependencies as sub-commands.