Refactor `RUN bash -xec` to just use `RUN`

This changes the `cd /usr/src/go/src` to use `WORKDIR` and `-x` flag to `|| exit $?`. I've tried this change on my machine, and [it seams fine](https://gist.github.com/maxnordlund/cb2f85901679cc9726fc). Fair warning the log is ~7600 lines long.