Improve Docker build

* Improves code readability.
* Improve documentation.
* Fix "docker stop" doesn't terminate qbt gracefully which could lead to
  data corruption.
* Provide correct/working bittorrent listening port by default.
* Make use of qbt profile option instead of hacking environment
  variables.
* Simplify build steps.

PR #16976.

dist/docker/Dockerfile

@@ -1,37 +1,51 @@
-
+# image for building
 FROM alpine:latest AS builder
 
-ARG BUILD_TYPE
-ARG RELEASE
+ARG QBT_VERSION
 
-RUN if [ $RELEASE = "master" ] ; \
-    then \
-      wget https://github.com/qbittorrent/qBittorrent/archive/refs/heads/master.zip && \
-      unzip master.zip && \
-      cd qBittorrent-master ; \
-    else \
-      wget https://github.com/qbittorrent/qBittorrent/archive/refs/tags/release-${RELEASE}.tar.gz && \
-      tar xf release-${RELEASE}.tar.gz && \
-      cd qBittorrent-release-${RELEASE} ; \
-    fi && \
-    apk add --no-cache qt6-qttools-dev g++ libtorrent-rasterbar-dev cmake boost-dev ninja && \
-    cmake -B build-nox -G "Ninja" -DCMAKE_BUILD_TYPE=$BUILD_TYPE -DGUI=OFF -DQT6=ON -DSTACKTRACE=OFF && \
-    cmake --build build-nox && \
-    cmake --build build-nox --target install/strip
+# alpine linux qbittorrent package: https://git.alpinelinux.org/aports/tree/community/qbittorrent/APKBUILD
 
+RUN \
+  apk --update-cache add \
+    boost-dev \
+    cmake \
+    g++ \
+    libtorrent-rasterbar-dev \
+    ninja \
+    qt6-qtbase-dev \
+    qt6-qttools-dev
+
+RUN \
+  if [ "$QBT_VERSION" = "devel" ]; then \
+    wget https://github.com/qbittorrent/qBittorrent/archive/refs/heads/master.zip && \
+    unzip master.zip && \
+    cd qBittorrent-master ; \
+  else \
+    wget "https://github.com/qbittorrent/qBittorrent/archive/refs/tags/release-${QBT_VERSION}.tar.gz" && \
+    tar -xf "release-${QBT_VERSION}.tar.gz" && \
+    cd "qBittorrent-release-${QBT_VERSION}" ; \
+  fi && \
+  cmake \
+    -B build \
+    -G Ninja \
+    -DCMAKE_BUILD_TYPE=RelWithDebInfo \
+    -DGUI=OFF \
+    -DQT6=ON \
+    -DSTACKTRACE=OFF && \
+  cmake --build build && \
+  cmake --install build
+
+# image for running
 FROM alpine:latest
 
+RUN \
+  apk --no-cache add \
+    libtorrent-rasterbar \
+    qt6-qtbase \
+    tini
+
 COPY --from=builder /usr/local/bin/qbittorrent-nox /usr/bin/qbittorrent-nox
 
 COPY entrypoint.sh /entrypoint.sh
 
-RUN chmod +x /entrypoint.sh && \
-    apk add --no-cache qt6-qtbase libtorrent-rasterbar
-
-ENV WEBUI_PORT="8080"
-
-EXPOSE 6881 6881/udp 8080
-
-VOLUME /config /data /downloads
-
-ENTRYPOINT ["/entrypoint.sh"]
+ENTRYPOINT ["/sbin/tini", "-g", "--", "/entrypoint.sh"]