Add gems remote type for RubyGems (#142)

Ruby builds fetch gems from rubygems.org directly because artifactapi has no RubyGems remote type.

- add `gems` package type proxying `https://rubygems.org`-style sources
- treat `versions`, `names`, `info/*` and `*specs.4.8.gz` as mutable, `gems/*.gem` and `quick/Marshal.4.8/*.gemspec.rz` as immutable
- serve `api/v1/*.json` as `application/json`
- add a gem/bundler usage snippet to the UI and a gems e2e caching case

Requires unkin/artifactapi#148 (HEAD route; RubyGems probes `HEAD /versions`).

Reviewed-on: #142
Co-authored-by: unkin-agent <unkin-agent@unkin.net>
Co-committed-by: unkin-agent <unkin-agent@unkin.net>

e2e-docker/README.md

@@ -23,7 +23,7 @@ already-running stack.
 
 - **Repository lifecycle** — add / change / delete for remote, local and virtual repos.
 - **Caching** — one immutable artifact per remote package type (generic, docker,
-  helm, pypi, npm, rpm, alpine, puppet, terraform, goproxy, cargo, nuget) proxied through the
+  helm, pypi, npm, rpm, alpine, puppet, terraform, goproxy, cargo, gems, nuget) proxied through the
   mock upstream: first fetch `X-Artifact-Source: remote`, second `cache`, bytes
   verified against the origin fixture.
 - **Local uploads** — generic (upload/download), pypi (wheel + generated `simple/`

e2e-docker/caching_test.go

@@ -30,6 +30,7 @@ func TestCachingPerProvider(t *testing.T) {
 		{"puppet", "puppet-releases/author-mod-1.0.0.tar.gz", "puppet-releases/author-mod-1.0.0.tar.gz"},
 		{"goproxy", "goproxy/example.com/mod/@v/v1.0.0.zip", "goproxy/example.com/mod/@v/v1.0.0.zip"},
 		{"cargo", "crates/mycrate/mycrate-1.0.0.crate", "crates/mycrate/mycrate-1.0.0.crate"},
+		{"gems", "gems/mygem-1.0.0.gem", "gems/mygem-1.0.0.gem"},
 		{"nuget", "nuget/e2epkg/1.0.0/e2epkg.1.0.0.nupkg", "nuget/e2epkg/1.0.0/e2epkg.1.0.0.nupkg"},
 		{"terraform", "hashicorp/aws/download/pkg.zip", "v1/providers/hashicorp/aws/download/pkg.zip"},
 		{"docker", "library/testimg/blobs/blobdata", "v2/library/testimg/blobs/blobdata"},

e2e-docker/fixtures/gems/mygem-1.0.0.gem

@@ -0,0 +1 @@
+mygem-1.0.0 e2e fixture