Add goproxy local repositories (#143)

Private Go modules have nowhere to live: goproxy is remote-only, so internal modules need VCS access or a hand-rolled proxy.

- serve local goproxy repos over the GOPROXY protocol (`@v/list`, `@latest`, `.info`/`.mod`/`.zip`) with case-encoded module paths
- accept PUT uploads of `.zip`/`.mod`/`.info` per version; synthesize `.info` when absent
- validate zips with `golang.org/x/mod/zip.CheckZip`, `.mod` module path and `.info` version before storing
- add a ContentValidator hook so providers can check upload bytes
- document publishing in the UI and `docs/goproxy/local.md`; add an e2e-docker case

Reviewed-on: #143
Co-authored-by: unkin-agent <unkin-agent@unkin.net>
Co-committed-by: unkin-agent <unkin-agent@unkin.net>

docs/goproxy/local.md

@@ -0,0 +1,20 @@
+# Local Go module proxy
+
+A local `goproxy` repo serves private modules over the GOPROXY protocol
+(`@v/list`, `@latest`, `@v/<ver>.info|.mod|.zip`). Publish a version by
+uploading its files under the case-encoded module path (uppercase as `!lower`):
+
+```sh
+for ext in zip mod info; do
+  curl -fsSL --upload-file v0.1.0.$ext \
+    https://artifactapi.k8s.syd1.au.unkin.net/api/v2/remotes/go-internal/files/example.unkin.net/hello/@v/v0.1.0.$ext
+done
+GOPROXY=https://artifactapi.k8s.syd1.au.unkin.net/api/v1/local/go-internal,https://proxy.golang.org,direct \
+GONOSUMDB=example.unkin.net go get example.unkin.net/hello@latest
+```
+
+Build the zip with `golang.org/x/mod/zip` or reuse the files `go mod download`
+leaves in `$(go env GOMODCACHE)/cache/download`. Uploads are validated: zips must
+pass `zip.CheckZip` (every file under `module@version/`), `.mod` must declare the
+module, and `.info` must match the version. `.info` is optional; it is
+synthesized from the version when absent. `@v/list` lists versions with a `.mod`.