Support a Docker image for develop

[turt2live]

We now have a Docker image autobuilding for releases and master, but don't have one for develop. There's a couple ways we can build this:

• Fix our release process to ensure that matrix-react-sdk and matrix-js-sdk get added to the package.json (and yarn lockfile) as develop, then enable autobuilds in Docker Hub.
• Set up a Buildkite job to build/publish the image for us, using build args.
• Use multiple Dockerfiles which change the build arguments, resulting in double the maintenance effort.

Ideally we'd just use build arguments in Docker Hub, however that is not supported: docker/hub-feedback#508