Use docker push to push image

[yuvipanda]

docker push has a couple of advantages over --push in repo2docker:

• If we use --push, we can not actually run the docker image-tests/ before pushing! So if an image-test/ fails, the docker image is still pushed! This is very confusing behavior.
• It times out sometimes, causing issues where the image just fails to push after building. docker push doesn't have this issue - see https://github.com/2i2c-org/utoronto-image/issues/20, where we had to manually run a docker push.
• docker push provides more useful output than --push

This commit also removes some code duplication - we now build the image regardless, and use the flag just to differentiate pushing. An unused bash function was also deleted.

[yuvipanda]

Tested to work with https://github.com/2i2c-org/utoronto-image/pull/24

[yuvipanda]

How do you feel about hitting the merge button, @hamelsmu? :)

[yuvipanda]

Thanks a lot, @hamelsmu!