Switch Dockerfile from CMD to ENTRYPOINT so that flags can be passed
to the makemigrations command
This is required in cases where we need to use django's makemigrations
--merge to merge two migrations.
* Adds a laptop-mode-env file which docker can read env variables from
* Adds a Dockerfile.makemigrations; mostly identical to the main
Dockerfile. Important difference: instead of the source being copied
into the docker image at build time, it's mounted from the local
machine at run time.
* Adds a makemigrations shell script which builds an imagine using the
Dockefile.makemigrations and then uses it to run makemigrations
* Because the source is mounted from the local machine, any new
migrations created are dumped in the developer's git checkout ready
for adding to git.
Rename .env -> docker/laptop-mode-env
