Add local gems repositories (#153)

In-house gems have nowhere to publish: gems locals only take raw file uploads, which `gem push`, `gem install` and bundler cannot use.

- accept `gem push` (`POST <repo>/api/v1/gems`, 256MiB cap, 8MiB metadata cap) and `gem yank` (`DELETE <repo>/api/v1/gems/yank`); any API key works
- parse the gemspec from the `.gem`; reject an existing name-version-platform with 409
- serve the compact index (`versions`, `info/<gem>`, `names`), `.gem` files and generated `quick/Marshal.4.8` specs; no legacy `specs.4.8.gz`
- route HEAD on locals (gem probes `versions` with HEAD)
- add `docs/gems/local.md`, a UI snippet and a stock `ruby:3.3` e2e-docker case

Reviewed-on: #153
Co-authored-by: unkin-agent <unkin-agent@unkin.net>
Co-committed-by: unkin-agent <unkin-agent@unkin.net>

ui/src/packagetypes/defs/gems.ts

@@ -2,7 +2,26 @@ import type { PackageTypeUI } from '../types';
 
 const def: PackageTypeUI = {
   color: 'red',
-  snippets: ({ proxy }) => {
+  snippets: ({ name, url, proxy, isLocal }) => {
+    if (isLocal) {
+      const src = `${url}/api/v1/local/${name}`;
+      return [
+        {
+          title: 'Push and yank',
+          language: 'bash',
+          code: `GEM_HOST_API_KEY=unused gem push --host ${src} <gem>-<version>.gem
+GEM_HOST_API_KEY=unused gem yank <gem> -v <version> --host ${src}`,
+          note: 'gem push needs an API key; any value works. Pushing an existing version is rejected.',
+        },
+        {
+          title: 'Install',
+          language: 'bash',
+          code: `gem install <gem> --source ${src}/
+# Gemfile
+source "${src}/"`,
+        },
+      ];
+    }
     return [
       {
         title: 'Use this remote as the RubyGems source',