Improve docker cache utilization on image construction (#1099)

To provide better use of Docker cache during image build by installing from PIP before creating the layer with the source code (the PIP installations are less probable to change than the rest of the source code, so this layer will not be recreated with all the source code changes)

This was previously discussed in #1094

Dockerfile

@@ -35,6 +35,22 @@ RUN apt-get update && \
 ENV LC_ALL=C.UTF-8 \
     DJANGO_SETTINGS_MODULE=production_settings
 
+# To copy only the requirements files needed to install from PIP
+COPY src/requirements /pretix/src/requirements
+COPY src/requirements.txt /pretix/src
+RUN pip3 install -U \
+        pip \
+        setuptools \
+        wheel && \
+    cd /pretix/src && \
+    pip3 install \
+        -r requirements.txt \
+        -r requirements/memcached.txt \
+        -r requirements/mysql.txt \
+        -r requirements/redis.txt \
+        gunicorn && \
+    rm -rf ~/.cache/pip
+
 COPY deployment/docker/pretix.bash /usr/local/bin/pretix
 COPY deployment/docker/supervisord.conf /etc/supervisord.conf
 COPY deployment/docker/nginx.conf /etc/nginx/nginx.conf
@@ -43,18 +59,8 @@ COPY src /pretix/src
 
 RUN chmod +x /usr/local/bin/pretix && \
     rm /etc/nginx/sites-enabled/default && \
-    pip3 install -U \
-        pip \
-        setuptools \
-        wheel && \
     cd /pretix/src && \
     rm -f pretix.cfg && \
-    pip3 install \
-        -r requirements.txt \
-        -r requirements/memcached.txt \
-        -r requirements/mysql.txt \
-        -r requirements/redis.txt \
-        gunicorn && \
 	mkdir -p data && \
     chown -R pretixuser:pretixuser /pretix /data data && \
 	sudo -u pretixuser make production