config:set no longer appears to work for Dockerfile-based apps

[vdice]

For Dockerfile-based apps, deis config:set -a <app> no longer appears to work:

vdice in example-dockerfile-http on (master) $ deis config:set PORT=7777 -a edh

vdice in example-dockerfile-http on (master) $ git push edh master
Counting objects: 102, done.
Delta compression using up to 4 threads.
Compressing objects: 100% (69/69), done.
Writing objects: 100% (102/102), 34.45 KiB | 0 bytes/s, done.
Total 102 (delta 31), reused 81 (delta 21)
remote: Resolving deltas: 100% (31/31), done.
Starting build... but first, coffee!
Step 1 : FROM alpine:3.5
 ---> 88e169ea8f46
Step 2 : RUN apk add -U 	bash 	nginx 	&& rm -rf /var/cache/apk*
 ---> Using cache
 ---> 25e99f30cea6
Step 3 : RUN ln -sf /dev/stdout /var/log/nginx/access.log
 ---> Using cache
 ---> d133753e0a3b
Step 4 : RUN ln -sf /dev/stderr /var/log/nginx/error.log
 ---> Using cache
 ---> 17fb26aabf03
Step 5 : RUN mkdir -p /run/nginx
 ---> Using cache
 ---> 9319a5d00649
Step 6 : RUN echo Кубернетис в каждый датацентр!
 ---> Using cache
 ---> e85f2d27892a
Step 7 : ENV POWERED_BY Deis
 ---> Using cache
 ---> b23c431a56c1
Step 8 : COPY rootfs /
 ---> Using cache
 ---> 8878fb9e8ce3
Step 9 : CMD /bin/boot
 ---> Using cache
 ---> 04258cb91628
Step 10 : EXPOSE 7777
 ---> Using cache
 ---> b01b00e93c21
One or more build-args [PORT] were not consumed, failing build.
remote: 2017/03/01 21:51:51 Error running git receive hook [Build pod exited with code 1, stopping build.]
To ssh://git@deis-builder.130.211.223.241.nip.io:2222/edh.git
 ! [remote rejected] master -> master (pre-receive hook declined)
error: failed to push some refs to 'ssh://git@deis-builder.130.211.223.241.nip.io:2222/edh.git'

Source of error: https://github.com/docker/docker/blob/master/docs/reference/builder.md#arg

Perhaps related: deis/dockerbuilder#117

If hunch is correct, either app/Dockerfile maintainer would need to add ARG foo for each env var attempting to be set via config:set or builder would need to do similar behind the scenes...

[bacongobbler]

Just tested and subsquent ARG foo entries work in a Dockerfile, so I believe asking the builder to do this in the background is a good approach to solve this problem.