To prevent unwanted surprises with new minor versions of go, lock the
version to minor version using [semver tilde
syntax](https://github.com/npm/node-semver#tilde-ranges-123-12-1). We
were already getting 1.21.0 since yesterday, so use that version now as
minimum.
The [docker/build-push-action@v2
action](https://github.com/docker/build-push-action) by default ignores
the checkout created using the actions/checkout@v2 action. When you pass
a git build context to docker build, it wouldn't include the .git
directory.
By passing `context: .` to the build step then it'll use the Actions git
context which includes the git fetch from the earlier step.
