Run the devcontainer in its own container - #3105

`init-dev-docker` ran `make dist && ./listmonk --install`, and was an explicit manual step that users would have to do to get the `dev-docker` environment up and running.

For users of the `dev-docker` development environment, this commit simplifies setup; users just have to `make dev-docker` without having to remember to `make init-dev-docker`.

But, more importantly, this fixes the broken `devcontainer.json`, which gave users no opportunity to `init-dev-docker` before it automatically tries to start the `backend` container.

Now, `backend` depends on a new `init` container, which runs `make init-docker` and exits. `make init-docker` runs `make build-frontend` (we never needed a production `make dist` build for development), builds `listmonk`, and runs `./listmonk --install --idempotent`.

On my machine, this imperceptibly slows down `dev-docker` startup, running `./listmonk --install --idempotent` before starting the `backend`.

Fixes knadh#3101