This replaces the docker image so that we get one that has all of these
commands available: make, git, go, python2 and perl.

We can now run `make generate` in the steps used to generate the release patch,
so the release patch contains the builtin_metadata.json changes.

Before, we had been running `make generate` in the working directory, and
copied the file into the container's workdir; but that left the working
directory modified, and hindered patch application before cleaning up.

Signed-off-by: Stephan Renatus <stephan.renatus@gmail.com>