Update Dockerfile with build-time optimisations

thallium-backend/Dockerfile

@@ -1,9 +1,5 @@
 FROM python:3.12-slim
 
-# Define Git SHA build argument for sentry
-ARG git_sha="development"
-ENV GIT_SHA=$git_sha
-
 RUN apt update -y \
  && apt install -y curl \
  && rm -rf /var/lib/apt/lists/*
@@ -13,10 +9,13 @@ WORKDIR /thallium
 COPY requirements.txt ./
 RUN pip install -r requirements.txt
 
-# Copy the source code in last to optimize rebuilding the image
 COPY . .
 
 HEALTHCHECK --start-period=5s --interval=30s --timeout=1s CMD curl http://localhost/heartbeat || exit 1
 
+# Define Git SHA build argument for sentry
+ARG git_sha="development"
+ENV GIT_SHA=$git_sha
+
 ENTRYPOINT ["/bin/bash", "-c"]
 CMD ["alembic upgrade head && uvicorn src.app:fastapi_app --host 0.0.0.0 --port 8000 --no-server-header"]