do not cache the service worker

As the contents of the service worker vary depending on if the user is
logged in don't cache it.

CHANGELOG.md

@@ -33,6 +33,7 @@
         - Fix a few obscure asset layer changing issues.
         - Fix multiple disable messages for dropdown answers
         - Do not trigger duplicate check when checking stoppers
+        - Do not cache the service worker
     - Admin improvements:
         - Display user name/email for contributed as reports. #2990
         - Interface for enabling anonymous reports for certain categories. #2989

perllib/FixMyStreet/App/Controller/Offline.pm

@@ -22,6 +22,7 @@ Offline pages Catalyst Controller - service worker handling
 
 sub service_worker : Path("/service-worker.js") {
     my ($self, $c) = @_;
+    $c->res->headers->header('Cache-Control' => 'max-age=0');
     $c->res->content_type('application/javascript');
 }
 

t/app/controller/offline.t

@@ -54,6 +54,7 @@ FixMyStreet::override_config {
 
 subtest 'service worker' => sub {
     $mech->get_ok('/service-worker.js');
+    is $mech->res->header('Cache-Control'), 'max-age=0', 'service worker is not cached';
     $mech->content_contains('translation_strings');
     $mech->content_contains('offline/fallback');
 };