Optimize CI and dockerfile caching

Dockerfile

@@ -5,33 +5,36 @@ RUN cd /frontend && yarn --prod && yarn build
 
 FROM alpine:3.11
 
-ENV UID=1337 \
-    GID=1337
+COPY requirements.txt /opt/maubot/requirements.txt
+WORKDIR /opt/maubot
+RUN apk add --no-cache --virtual .build-deps \
+        python3-dev \
+        build-base \
+        git \
+    && apk add --no-cache \
+        ca-certificates \
+        su-exec \
+        py3-aiohttp \
+        py3-sqlalchemy \
+        py3-attrs \
+        py3-bcrypt \
+        py3-cffi \
+        py3-pillow \
+        py3-magic \
+        py3-psycopg2 \
+        py3-ruamel.yaml \
+        py3-jinja2 \
+        py3-click \
+        py3-packaging \
+        py3-markdown \
+    && pip3 install -r requirements.txt \
+        feedparser dateparser langdetect python-gitlab \
+    && apk del .build-deps
+# TODO remove pillow, magic and feedparser when maubot supports installing dependencies
 
 COPY . /opt/maubot
 COPY --from=frontend-builder /frontend/build /opt/maubot/frontend
-WORKDIR /opt/maubot
-RUN apk add --no-cache \
-      py3-aiohttp \
-      py3-sqlalchemy \
-      py3-attrs \
-      py3-bcrypt \
-      py3-cffi \
-      build-base \
-      python3-dev \
-      ca-certificates \
-      su-exec \
-      py3-pillow \
-      py3-magic \
-      py3-psycopg2 \
-      py3-ruamel.yaml \
-      py3-jinja2 \
-      py3-click \
-      py3-packaging \
-      py3-markdown \
-      && pip3 install -r requirements.txt feedparser dateparser langdetect python-gitlab
-# TODO remove pillow, magic and feedparser when maubot supports installing dependencies
-
+ENV UID=1337 GID=1337
 VOLUME /data
 
 CMD ["/opt/maubot/docker/run.sh"]