[#246] Update backend Dockerfile to use base image

This commit updates the Dockerfile for the GovTool backend to utilize
the newly created base image, which contains all necessary pre-compiled
Haskell dependencies. By changing the base image from
`haskell:9.2-buster` to the custom `backend-base` image hosted on AWS
ECR, the build process for the backend Docker image is significantly
optimized.

The key changes include:
- Modifying the FROM directive to point to the `backend-base` image,
  ensuring that the environment is pre-setup with all the dependencies
  required for the backend build.
- Simplifying the build command to `cabal build` since `cabal update`,
  `cabal configure`, and dependency installations are no longer
  necessary, being already handled in the base image creation process.

This adjustment ensures that the backend Dockerfile leverages the
pre-compiled dependencies in the base image, directly contributing to a
faster build and deployment process. This change aligns with the
project's acceptance criteria by ensuring that only the
application-specific code is compiled during the build process,
leveraging the efficiencies offered by the base image.

govtool/backend/Dockerfile

@@ -1,5 +1,5 @@
-FROM haskell:9.2-buster
+FROM 733019650473.dkr.ecr.eu-west-1.amazonaws.com/backend-base:39906fb
 WORKDIR /src
 COPY . .
-RUN cabal update && cabal configure && cabal build
+RUN cabal build
 RUN cp dist-newstyle/build/x86_64-linux/ghc-9.2.8/vva-be-0.1.0.0/x/vva-be/build/vva-be/vva-be /usr/local/bin