Add `.dockerignore` file to avoid copying unneeded build components

[griff-rees]

The COPY . . lines in compose/local/django/Dockerfile and compose/production/django/Dockerfile copy everything in the local directory, including, for example, extra database files left in the backups folder that aren't needed for the local deploy. There are advantages to this (automatically adding files needed) but as more and more files are added locally this may be more difficult to maintain. The easiest to solution is to include a .dockerignore file, but a more robust would be to specify what gets copied like: https://stackoverflow.com/questions/43747776/copy-with-docker-but-with-exclusion

[griff-rees]

Addressed in 734599a will close when merged.

[griff-rees]

Addressed in merge 91fd36996a3efa0227bc9b009b99f428882e103b