Optimize CI and dockerfile caching

Dockerfile.ci

@@ -0,0 +1,34 @@
+FROM alpine:3.11
+
+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
+ENV UID=1337 GID=1337
+VOLUME /data
+
+CMD ["/opt/maubot/docker/run.sh"]