[Packaging] Optimize image size of govtool/backend

[mesudip]

The current docker image size is of 5.39GB which is very large for a simple service, compared to cardano-node or db-sync.

Image size reported for different docker images

REPOSITORY                                         TAG       IMAGE ID       CREATED         SIZE
govtool/backend                                    latest    b6413eb58785   17 hours ago    5.39GB
govtool/backend-base                               latest    3a852d2c9dc2   22 hours ago    5.21GB
ghcr.io/intersectmbo/cardano-node                  <none>    ddba06d70271   7 months ago    911MB
ghcr.io/intersectmbo/cardano-node                  <none>    aef9e1df664f   7 months ago    928MB

Suggestions

• Separate out docker image for build and runtime with multi-stage docker file.
• Exclude ghc in the final image. ghc build system is very large (~2Gb) this is not required at runtime.
• Exclude cabal store and packages in final image.
• Exclude dev libraries in final image e.g libpng-dev libpq-dev libreadline-dev libpsql-dev etc. These are only required for building backend with ghc and not during the runtime.