Continuous deployment to dockerhub

It would be useful to have a Github action which when a PR is merged, or change committed to master branch, builds the docker containers and if there are no errors, pushes the new images to https://hub.docker.com/u/intermine/.

The docker commands for doing this once auth secrets have been added: (filling in `imagename` and `commithash`)
```
docker build -t imagename path/to/Dockerfile
docker tag imagename intermine/imagename:latest
docker tag imagename intermine/imagename:commithash
docker push intermine/imagename
```
For the sake of automation, I think using the commit hash instead of a semantic version is okay.