feat: add release target to Makefile

I used the Gitlab CI of Framasoft to get the release code. See
5a30bc2f15/.gitlab-ci.yml (L280)

Makefile

@@ -25,3 +25,14 @@ format:
 	docker-compose run --rm api bash -c "mix format && mix credo --strict"
 	@bash docker/message.sh "Code is now ready to commit :)"
 target: init
+
+RELEASE_TAG=$(shell git tag --list '[0-9]*.[0-9]*.[0-9]*' | tail -n1)
+release:
+	mix local.hex --force
+	mix local.rebar --force
+	mix deps.get --only-prod
+	mix compile
+	mix phx.digest.clean --all && mix phx.digest
+	mix release --path release/mobilizon
+	cd release/mobilizon && ln -s lib/mobilizon-*/priv priv
+	tar czf mobilizon-$(RELEASE_TAG).tar.gz -C release mobilizon