@bacongobbler Won't this cause a rebuild of the docker image every time a new environment var is set on the app? Or is docker smart enough about that?

If a Dockerfile defines an ARG variable whose value is different from a previous build, then a “cache miss” occurs upon its first usage, not its definition. Since it's appended to the end of the Dockerfile then this won't cause cache misses.

@bacongobbler may need a minor formatting change to add a newline between multiple args:

$ deis config:set PORT=7777 DOO=FOO -a edh
Creating config... done

=== edh Config
DOO       FOO
PORT      7777

$ git push edh-mk master
...
Step 12 : ARG DOOARG PORT
ARG requires exactly one argument definition

Thanks for testing this! I even ensured that we write to a new line as well to ensure we're not clashing with existing dockerfiles.

Testing looks good; happy path spec around this added in deis/workflow-e2e#349 (didn't appear we previously had a spec that covered config:set on dockerfile app). The e2e run on that pr will run with this change as well.