docker: add comments

src/docker.js

@@ -170,8 +170,11 @@ async function downloadImage(manifest) {
     const parsedManifestRef = parseImageRef(manifest.dockerImage);
 
     await promiseRetry({ times: 10, interval: 5000, debug, retry: (pullError) => pullError.reason !== BoxError.FS_ERROR }, async () => {
+        // custom (non appstore) image
         if (parsedManifestRef.registry !== null || !parsedManifestRef.fullRepositoryName.startsWith('cloudron/')) return await pullImage(manifest.dockerImage);
 
+        // docker hub only uses first 64 bits for ipv6 addressing. this causes many ipv6 rate limit errors
+        // https://www.docker.com/blog/beta-ipv6-support-on-docker-hub-registry/ . as a hack, we try ipv4 explicity
         let upstreamRef = null;
         for (const registry of [ 'registry.docker.com', 'registry.ipv4.docker.com', 'quay.io' ]) {
             upstreamRef = `${registry}/${manifest.dockerImage}`;