Explain caching strategy in Dockerfile comments

Dockerfile

@@ -29,26 +29,34 @@ RUN pip install \
 # django-rq is used for webhooks
       django-rq
 
-WORKDIR /opt
-
 ARG BRANCH=master
+
+WORKDIR /tmp
+
+# As the requirements don't change very often,
+# and as they take some time to compile,
+# we try to cache them very agressively.
 ARG REQUIREMENTS_URL=https://raw.githubusercontent.com/digitalocean/netbox/$BRANCH/requirements.txt
 ADD ${REQUIREMENTS_URL} requirements.txt
 RUN pip install -r requirements.txt
 
+# Cache bust when the upstream branch changes:
+# ADD will fetch the file and check if it has changed
+# If not, Docker will use the existing build cache.
+# If yes, Docker will bust the cache and run every build step from here on.
 ARG REF_URL=https://api.github.com/repos/digitalocean/netbox/contents?ref=$BRANCH
 ADD ${REF_URL} version.json
 
+WORKDIR /opt
+
 ARG URL=https://github.com/digitalocean/netbox/archive/$BRANCH.tar.gz
 RUN wget -q -O - "${URL}" | tar xz \
   && mv netbox* netbox
 
-WORKDIR /opt/netbox
-
 COPY docker/configuration.docker.py /opt/netbox/netbox/netbox/configuration.py
 COPY configuration/gunicorn_config.py /etc/netbox/config/
 COPY docker/nginx.conf /etc/netbox-nginx/nginx.conf
-COPY docker/docker-entrypoint.sh docker-entrypoint.sh
+COPY docker/docker-entrypoint.sh /opt/netbox/docker-entrypoint.sh
 COPY startup_scripts/ /opt/netbox/startup_scripts/
 COPY initializers/ /opt/netbox/initializers/
 COPY configuration/configuration.py /etc/netbox/config/configuration.py