Allow flags for migrations

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.

Create a dev docker container for makemigrations

* 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