`run` should work with multiple lines

[bfirsh]

Currently run commands in the build section of can only be a single line. They should support multiple lines.

The sensible way of supporting this would probably be to write out the build commands to a shell script and then run that. E.g. write out string to tmpdir/build/0, tmpdir/build/1, etc, then output this in the Dockerfile:

COPY tmpdir/build /tmp/build
RUN /bin/sh /tmp/build/0
RUN /bin/sh /tmp/build/1

[bfirsh]

Oh, we can just do this: https://www.docker.com/blog/introduction-to-heredocs-in-dockerfiles/

With a suitable unique heredoc string.