Fix docker buildfile

[cflipse]

Was resulting in an error when attempting to run the build:
app_1 | /usr/local/bundle/gems/thor-1.0.1/lib/thor/parser/arguments.rb:133:in `parse_numeric': Expected numeric value for '--port'; got "${APP_PORT}" (Thor::MalformattedArgumentError)

There are two things that aren't quite set up right here:

The build arg was not in scope -- used args need to be below the
FROM statement to be accessible.

CMD appears to not allow variable expansion, because it is processed at
runtime rather than build time. Instead, we can export the PORT
envar, which will be picked up by config/puma.rb when starting rails.

[grenewode]

This all looks good! We should think about if there's a way to run ci tests on these so we avoid these kinds of breakages in the future!