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>

ui/src/packagetypes/defs/goproxy.ts

@@ -2,7 +2,31 @@ import type { PackageTypeUI } from '../types';
 
 const def: PackageTypeUI = {
   color: 'green',
-  snippets: ({ proxy }) => {
+  snippets: ({ name, url, proxy, isLocal }) => {
+    if (isLocal) {
+      return [
+        {
+          title: 'Fetch private modules from this proxy',
+          language: 'bash',
+          code: `export GOPROXY=${url}/api/v1/local/${name},https://proxy.golang.org,direct
+export GONOSUMDB=example.unkin.net
+go get example.unkin.net/hello@latest`,
+          note: 'Set GONOSUMDB (or GOPRIVATE) to your module prefix: private modules are not in sum.golang.org.',
+        },
+        {
+          title: 'Publish a module version',
+          language: 'bash',
+          code: `# build the zip with golang.org/x/mod/zip (CreateFromDir), or reuse
+# the .zip/.mod/.info from $(go env GOMODCACHE)/cache/download after go mod download
+MOD=example.unkin.net/hello VER=v0.1.0
+for ext in zip mod info; do
+  curl -fsSL --upload-file $VER.$ext \\
+    ${url}/api/v2/remotes/${name}/files/$MOD/@v/$VER.$ext
+done`,
+          note: 'Paths use the case-encoded module path (uppercase as !lower). Zips must hold every file under module@version/ and are rejected otherwise; .info is optional.',
+        },
+      ];
+    }
     return [
       {
         title: 'Point the Go module proxy here',