build_push_image: Add default Dockerfiles by language

With this addition, we can have "templates" of Dockerfiles for different languages, that will be fetched and used by CircleCI so they don't need to be present in the apps' repositories.
This makes sense because in 99% of the cases all our apps are using a fixed Dockerfile that we just copy around. So this should ease the rollout of newer versions of the Dockerfile.
Of course, it is still possible to provide a Dockerfile in the original app repo, if one exists that one will be used.

NOTE: if you want to use a Dockerfile from a template, you also need to remove the Dockerfile from your repository and add Dockerfile to the repo's .gitignore, to prevent Isopod from throwing a "working directory not clean" error.