flask: dev envs support & misc improvements (#263)

* Docker Desktop Development Environments config
* Use cache volumes for pip
* Upgrade from Python 3.7 -> Python 3.10
* Use port `8000` to avoid conflicts with Airplay on
  macOS for default Flask port `5000`
* Use `SIGINT` to gracefully stop Flask

Signed-off-by: Milas Bowman <milas.bowman@docker.com>

flask/app/Dockerfile

@@ -1,7 +1,27 @@
-FROM python:3.7-alpine 
-WORKDIR /app 
+# syntax=docker/dockerfile:1.4
+FROM --platform=$BUILDPLATFORM python:3.10-alpine AS builder
+
+WORKDIR /app
+
 COPY requirements.txt /app
-RUN pip3 install -r requirements.txt --no-cache-dir
-COPY . /app 
-ENTRYPOINT ["python3"] 
+RUN --mount=type=cache,target=/root/.cache/pip \
+    pip3 install -r requirements.txt
+
+COPY . /app
+
+ENTRYPOINT ["python3"]
 CMD ["app.py"]
+
+FROM builder as dev-envs
+
+RUN <<EOF
+apk update
+apk add git
+EOF
+
+RUN <<EOF
+addgroup -S docker
+adduser -S --shell /bin/bash --ingroup docker vscode
+EOF
+# install Docker tools (cli, buildx, compose)
+COPY --from=gloursdocker/docker / /