proxy through varnish

sets up varnish cache server with:
- custom cache per content type
- gzip
- configure app & hass backends
- varnish debug headers & X-Cache verbose cache header

also updates docker-compose, varnish/Dockerfile & drone.

varnish/vcl_deliver.vcl

@@ -0,0 +1,40 @@
+sub vcl_deliver {
+    # Happens when we have all the pieces we need, and are about to send the
+    # response to the client.
+
+    if (resp.status == 503) {
+        set resp.http.failing-backend = "true";
+    }
+
+    # Give some debug
+    if (req.http.X-debug && req.esi_level == 0) {
+        set resp.http.X-Backend = req.backend_hint;
+        set resp.http.X-Backend-Url = req.url;
+    } else {
+        # not debug, strip some headers
+        unset resp.http.X-Cache;
+        unset resp.http.X-Backend;
+        unset resp.http.x-upstream;
+        unset resp.http.x-request-uri;
+        unset resp.http.Via;
+        unset resp.http.xkey;
+        unset resp.http.x-goog-hash;
+        unset resp.http.x-goog-generation;
+        unset resp.http.X-GUploader-UploadID;
+        unset resp.http.x-goog-storage-class;
+        unset resp.http.x-goog-metageneration;
+        unset resp.http.x-goog-stored-content-length;
+        unset resp.http.x-goog-stored-content-encoding;
+        unset resp.http.x-goog-meta-goog-reserved-file-mtime;
+        unset resp.http.Server;
+        unset resp.http.X-Apache-Host;
+        unset resp.http.X-Varnish-Backend;
+        unset resp.http.X-Varnish-Host;
+        unset resp.http.X-Nginx-Host;
+        unset resp.http.X-Upstream-Age;
+        unset resp.http.X-Retries;
+        unset resp.http.X-TTL;
+        unset resp.http.X-Varnish;
+    }
+}
+