Allow bypassing docker build in the build.rs file for quicker builds.

[stonecharioteer]

This is to address this Pull Request's review notes. There needs to be a way to disable the docker build steps. This isn't straightforward sadly, since it is not possible right now to detect whether cargo is running build or test. So I think we can either do this using an environment variable (I'm strongly leaning towards this), or to write a Makefile (I hate makefiles, and they get complicated way too fast.

@noot the two options would be:

BUILD_DOCKER=1 cargo test

or

make tests

I think the first option will be cleaner and simpler to understand.