Use commit hash from last successful build as `GIT_DIFF`

At the moment `docker-build` and `docker-push` use the default `@~..@` for the env var `GIT_DIFF`. This means that only the last commit will be used to find out what to build and what to push.

We should use the commit hash from the last successful build in the `master` branch as part of `GIT_DIFF`.