[#246] Make backend base image version dependent on cabal file

This commit introduces a significant enhancement to the Docker build
process for the backend by making the base image versioning dependent on
the `vva-be.cabal` file. This change ensures that the base Docker image
is closely tied to the project's Haskell dependencies, as specified in
the cabal file.

- In `govtool/backend/Dockerfile`, an `ARG BASE_IMAGE_TAG` is
  introduced, allowing for dynamic specification of the base image tag
  during the build process. This facilitates using different versions of
  the base image as dependencies evolve.
- The `scripts/govtool/Makefile` is updated to calculate the
  `base_backend_image_tag` by hashing the `vva-be.cabal` file. This hash
  becomes the tag for the base image, ensuring that any changes in the
  cabal file result in a new base image version. This mechanism automates
  the versioning process, ensuring that the backend is always built
  against the correct set of pre-compiled dependencies.

By linking the base image version directly to the cabal file's hash,
this approach guarantees that any changes in dependencies are
automatically accounted for, thereby streamlining the build process and
enhancing consistency across builds.

govtool/backend/Dockerfile

@@ -1,4 +1,5 @@
-FROM 733019650473.dkr.ecr.eu-west-1.amazonaws.com/backend-base:39906fb
+ARG BASE_IMAGE_TAG
+FROM 733019650473.dkr.ecr.eu-west-1.amazonaws.com/backend-base:$BASE_IMAGE_TAG
 WORKDIR /src
 COPY . .
 RUN cabal build

scripts/govtool/Makefile

@@ -22,6 +22,7 @@ docker_user := ubuntu
 ssh_url := $(docker_user)@$(docker_host)
 docker_compose_file := docker-compose.$(env).yml
 compose_stack_name := govtool-$(env)-$(cardano_network)
+base_backend_image_tag := $(shell git hash-object ../../govtool/backend/vva-be.cabal)
 
 # helper function for checking undefined variables
 check_defined = \
@@ -67,14 +68,14 @@ upload-config: prepare-config
 build-backend:
 	@:$(call check_defined, cardano_network)
 	@:$(call check_defined, env)
-	$(docker) build --tag "$(repo_url)/backend:$(tag)" ../../govtool/backend
+	$(docker) build --build-arg BASE_IMAGE_TAG=$(base_backend_image_tag) --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)"
+	$(docker) build --file ../../govtool/backend/Dockerfile.base --tag "$(repo_url)/backend-base:$(base_backend_image_tag)" ../../govtool/backend
+	@echo "New backend-base image: $(repo_url)/backend-base:$(base_backend_image_tag)"
 
 .PHONY: push-backend
 push-backend: docker-login
@@ -86,7 +87,7 @@ push-backend: docker-login
 push-backend-base: docker-login
 	@:$(call check_defined, cardano_network)
 	@:$(call check_defined, env)
-	$(docker) push $(repo_url)/backend-base:$(commit)
+	$(docker) push $(repo_url)/backend-base:$(base_backend_image_tag)
 
 .PHONY: build-frontend
 build-frontend: