Bug 1085631 part2: Allow the use of a custom build script.

If the image folder contains an inner build.sh script, runs it to build
the image. The tag value is passed through a "-t" argument. All
arguments passed after the folder parameter are forwarded to the inner
build script.
Bug 1085631 part3: Add phone builder docker image.

As this image handles files containing partners blobs, it must be
private in quay.io, and the builds cannot be made public either.

You must use "-i" and "-k" build parameters to handle s3 credentials.