chore: update Haskell Dockerfile for improved caching and dependency management

- Added environment variable to ensure container rebuilds when dependency files change.
- Adjusted COPY commands to prevent cache misses by excluding .git and README.md.
- Removed unnecessary dummy content and streamlined the build process.

Author: andy1li

dockerfiles/haskell-9.8.Dockerfile

@@ -1,6 +1,7 @@
 FROM haskell:9.8.4-bullseye
 
-WORKDIR /app
+# Ensures the container is re-built if go.mod or go.sum changes
+ENV CODECRAFTERS_DEPENDENCY_FILE_PATHS="stack.yaml,package.yaml,stack.yaml.lock"
 
 RUN mkdir -p /etc/stack
 
@@ -11,24 +12,17 @@ RUN echo "allow-different-user: true" >> /etc/stack/config.yaml
 RUN echo "install-ghc: false" >> /etc/stack/config.yaml
 RUN echo "system-ghc: true" >> /etc/stack/config.yaml
 
-COPY stack.yaml package.yaml stack.yaml.lock /app/
+WORKDIR /app
 
-# Dummy static content to circumvent the /app doesn't exist warning
-RUN mkdir /app/app
-RUN echo 'main :: IO ()' >> /app/app/Main.hs
-RUN echo 'main = putStrLn "Hello, World!"' >> /app/app/Main.hs
+# .git & README.md are unique per-repository. We ignore them on first copy to prevent cache misses
+COPY . /app/
+RUN rm -rf /app/.git /app/README.md
 
 ENV STACK_ROOT=/app/.stack
-
 RUN stack build
 RUN stack clean codecrafters-http-server
+
 RUN mkdir /app-cached
 RUN mv .stack-work /app-cached/.stack-work
 RUN mv .stack /app-cached/.stack
-
 RUN rm -rf /app/app
-
-RUN echo "cd \${CODECRAFTERS_REPOSITORY_DIR} && stack build" > /codecrafters-precompile.sh
-RUN chmod +x /codecrafters-precompile.sh
-
-ENV CODECRAFTERS_DEPENDENCY_FILE_PATHS="stack.yaml,package.yaml,stack.yaml.lock"