Add caching directive for static content

Images and other static assets that are currently not cache-busted
will be cached for a week.

CSS and JS files (which we currently cache-bust) can be cached for
an arbitrarily long time, so we set it to a year.

This change fixes F grades on performance test sites in the
"Cache static content" category. Now we get an A!

NOTE: No particularly strong reason why `mod_headers` was used to set
the "max-age" header instead of `mod_expires`.

Author: hemberger

Dockerfile

@@ -53,6 +53,10 @@ RUN rm -rf /var/www/html/ && ln -s "$(pwd)/htdocs" /var/www/html
 # Make the upload directory writable by the apache user
 RUN chown www-data ./htdocs/upload
 
+# Leverage browser caching of static assets using apache's mod_headers
+COPY apache/cache-static.conf /etc/apache2/conf-enabled/cache-static.conf
+RUN a2enmod headers
+
 # Store the git commit hash of the repo in the final image
 COPY .git/HEAD .git/HEAD
 COPY .git/refs .git/refs

apache/cache-static.conf

@@ -0,0 +1,9 @@
+# Static assets (with no cache-bust), cache for 1 week
+<FilesMatch "\.(ico|jpg|png|gif|svg|woff)$">
+  Header set Cache-Control "max-age=604800, public"
+</FilesMatch>
+
+# Static assets (with cache-bust), cache for 1 year
+<FilesMatch "\.(js|css)$">
+  Header set Cache-Control "max-age=31536000, public"
+</FilesMatch>