[#246] Add steps for building and pushing base backend image

This commit enhances the `scripts/govtool/Makefile` by introducing new targets `build-backend-base` and `push-backend-base`. These additions facilitate the process of building and pushing the base Docker image for the backend, which contains all necessary pre-compiled dependencies to expedite subsequent builds.

- The `build-backend-base` target is responsible for building the base image using the `Dockerfile.base` located in the `govtool/backend` directory. It tags the image with the commit hash, providing a clear linkage between the image and the source code state from which it was built.
- The `push-backend-base` target, dependent on a successful Docker login (`docker-login`), pushes the newly built base image to the configured repository. This ensures that the image is available for use by the CI/CD pipelines and developers for building the actual backend application image.

These targets are crucial for implementing the strategy to speed up the backend build and deployment process by leveraging a Docker image with pre-compiled dependencies. This approach aligns with the acceptance criteria set forth for the project, aiming at reducing build and deployment times through efficient dependency management.

scripts/govtool/Makefile

@@ -69,12 +69,25 @@ build-backend:
 	@:$(call check_defined, env)
 	$(docker) build --tag "$(repo_url)/backend:$(tag)" ../../govtool/backend
 
+.PHONY: build-backend-base
+build-backend-base:
+	@:$(call check_defined, cardano_network)
+	@:$(call check_defined, env)
+	$(docker) build --file ../../govtool/backend/Dockerfile.base --tag "$(repo_url)/backend-base:$(commit)" ../../govtool/backend
+	@echo "New backend-base image: $(repo_url)/backend-base:$(commit)"
+
 .PHONY: push-backend
 push-backend: docker-login
 	@:$(call check_defined, cardano_network)
 	@:$(call check_defined, env)
 	$(docker) push $(repo_url)/backend:$(tag)
 
+.PHONY: push-backend-base
+push-backend-base: docker-login
+	@:$(call check_defined, cardano_network)
+	@:$(call check_defined, env)
+	$(docker) push $(repo_url)/backend-base:$(commit)
+
 .PHONY: build-frontend
 build-frontend:
 	@:$(call check_defined, cardano_network)