[#246] Add base Dockerfile for Haskell backend dependencies

Introduces a new Dockerfile.base within the govtool/backend directory,
aimed at creating a base image for the Haskell backend project. This
base image includes all necessary pre-compiled dependencies, following
the initiative to expedite the build process for deployment.

The Dockerfile is based on the `haskell:9.2-buster` image and sets up an
environment optimized for the GovTool project by pre-installing all
dependencies. By removing the source code after dependency installation,
we ensure that the image only contains what's necessary for future
builds, thus aligning with our goal to reduce build and deployment times
significantly.

This change addresses part of the acceptance criteria for speeding up
the deployment process by preparing and maintaining a base image with
all pre-compiled dependencies required by the application.

govtool/backend/Dockerfile.base

@@ -0,0 +1,10 @@
+# NOTE: This Dockerfile sets up an environment with precompiled dependencies for
+# the GovTool Haskell backend project, streamlining the project's compilation
+# process by ensuring it only needs to compile against these dependencies. This
+# is a common practice in Haskell projects, as it can significantly reduce the
+# time it takes to build the project.
+
+FROM haskell:9.2-buster
+WORKDIR /src
+COPY . .
+RUN cabal update && cabal configure && cabal install --only-dependencies && rm -rf /src/*