Add gems virtual repositories (#154)

Bundler needs one source for internal and public gems, but gems virtuals have no merger, so a local gems repo and rubygems.org cannot be served together.

- merge `info/<gem>` across members (member order wins a duplicate version-platform); pass a single-member gem through verbatim
- regenerate `versions` lines and md5s for gems several members list; union `names`
- 302 `gems/*.gem` and `quick/Marshal.4.8/*` to the member owning that version, via the shared member-path validation
- return 502 on a member error; cache merges in the shared virtual cache; route HEAD on virtuals
- add `docs/gems/virtual.md` and a stock `ruby:3.3` bundler e2e-docker case

Reviewed-on: #154
Co-authored-by: unkin-agent <unkin-agent@unkin.net>
Co-committed-by: unkin-agent <unkin-agent@unkin.net>

docs/gems/virtual.md

@@ -0,0 +1,28 @@
+# `gems` virtual
+
+A gems virtual serves one RubyGems source over local and remote gems members:
+`gem install` and `bundle install` work against `/api/v1/virtual/<name>/`.
+
+- `info/<gem>` is the union of member lines; on a duplicate version-platform
+  the earlier member wins. A gem only one member has is served verbatim.
+- `versions` regenerates the line (and md5) of every gem several members list
+  from its merged `info`; other gems' lines pass through. `names` is the union.
+- `gems/*.gem` and `quick/Marshal.4.8/*.gemspec.rz` 302 to the member whose
+  `info` line won.
+- A member error fails the request with 502; a member without the gem is
+  skipped. Merges are cached for 60s.
+
+```hcl
+resource "artifactapi_virtual" "gems" {
+  name         = "gems"
+  package_type = "gems"
+  members      = [artifactapi_local_gems.internal.name, artifactapi_remote_gems.rubygems.name]
+}
+```
+
+```ruby
+# Gemfile
+source "https://artifactapi.example.com/api/v1/virtual/gems/"
+```
+
+Push to the local member directly; the virtual is read-only.