[build] Configure docker build and run arguments

When testing and building locally, I frequently find myself fiddling with the Avro docker image to try and reproduce what is happening on Travis.

We currently can specify additional docker run arguments using the build.sh docker --args "xxxx" format. I'd like to be able to specify:

• additional docker build arguments (usually --no-cache)
• the docker run entrypoint, other than bash (for scripting some tasks).

I don't think it's necessary to add additional arguments to the build.sh script, it's probably enough to use environment variables that can be set outside of the script. I propose:

• DOCKER_BUILD_XTRA_ARGS to add additional arguments to the docker image build.
• DOCKER_RUN_XTRA_ARGS (rename the existing DOCKER_XTRA_ARGS) to add additional arguments to the docker run.
• DOCKER_IMAGE_NAME to specify the docker image being used (currently avro-build-$USER_NAME:latest)
• DOCKER_RUN_ENTRYPOINT to specify the entrypoint being used (currently bash)

There should be no changes or impact in an environment that doesn't set these arguments.