build (Dockerfile.web): improve caching

- move source code beneath venv
- add deps stage for caching
- copy venv before source code in final stage

Author: pythoninthegrass

Dockerfile.web

@@ -18,7 +18,6 @@ RUN uv python install "$PYTHON_VERSION"
 
 WORKDIR /app
 
-COPY ./app .
 COPY pyproject.toml .
 
 # venv
@@ -29,7 +28,22 @@ ENV PATH="$VENV/bin:$PATH"
 RUN uv venv $UV_PROJECT_ENVIRONMENT \
     && uv pip install -r pyproject.toml
 
-FROM python:${PYTHON_VERSION}-slim-bookworm as runner
+COPY ./app .
+
+FROM python:${PYTHON_VERSION}-slim-bookworm as deps
+
+# avoid stuck build due to user prompt
+ARG DEBIAN_FRONTEND=noninteractive
+
+# install dependencies
+RUN apt-get -qq update \
+    && apt-get -qq install \
+    --no-install-recommends -y \
+    curl \
+    lsof \
+    && rm -rf /var/lib/apt/lists/*
+
+FROM deps as runner
 
 # set timezone
 ENV TZ=${TZ:-"America/Chicago"}
@@ -51,17 +65,6 @@ ENV PYTHONPATH=/app
 # https://github.com/tiangolo/uvicorn-gunicorn-fastapi-docker/blob/master/README.md#web_concurrency
 ENV WEB_CONCURRENCY=2
 
-# avoid stuck build due to user prompt
-ARG DEBIAN_FRONTEND=noninteractive
-
-# install dependencies
-RUN apt-get -qq update \
-    && apt-get -qq install \
-    --no-install-recommends -y \
-    curl \
-    lsof \
-    && rm -rf /var/lib/apt/lists/*
-
 # add non-root user
 ARG UID=10001
 RUN adduser \
@@ -80,9 +83,12 @@ COPY --from=builder --chown=${USER_NAME}:${USER_NAME} /python /python
 
 WORKDIR /app
 
-COPY --chown=${USER_NAME} ./app .
+# Copy venv first (has better caching properties)
 COPY --from=builder --chown=${USER_NAME} "$VENV" "$VENV"
 
+# Copy application code last
+COPY --chown=${USER_NAME} ./app .
+
 ARG PORT=${PORT:-3100}
 EXPOSE $PORT