Add alpine virtual repositories (#164)

Virtual repos cover rpm, deb, npm and others but not alpine, so a local alpine repo and github_alpine remotes cannot be served as one apk repository.

- merge members' `<dir>/APKINDEX.tar.gz` per directory; member order wins duplicate (P, V, A); a member 404 is skipped
- 302 `<dir>/<P>-<V>.apk` downloads to the member whose index listed the package
- serve the merged index unsigned, like local and github_alpine indexes
- cache merges per directory on the shared virtual cache; member errors fail with 502
- add docs/alpine/virtual.md, unit tests and an apk e2e with abuild-built packages

Requires unkin/artifactapi#163.

Reviewed-on: #164
Co-authored-by: unkin-agent <unkin-agent@unkin.net>
Co-committed-by: unkin-agent <unkin-agent@unkin.net>

docs/alpine/virtual.md

@@ -0,0 +1,30 @@
+# `alpine` virtual
+
+An alpine virtual serves one apk repository over local and remote alpine
+members at `/api/v1/virtual/<name>/`.
+
+- `<dir>/APKINDEX.tar.gz` merges each member's index for the same `<dir>`
+  (e.g. `x86_64/`); on a duplicate (P, V, A) the earlier member wins. A member
+  that 404s for that directory is skipped.
+- apk builds download paths itself (`<dir>/<P>-<V>.apk`), so the virtual 302s
+  each download to the member whose index listed it.
+- The merged index is unsigned, like local and `github_alpine` indexes: clients
+  need `--allow-untrusted` (packages keep their own signatures).
+- Any other member error fails the request with 502. Merges are cached for 60s
+  per directory, up to 32 directories per virtual.
+
+```hcl
+resource "artifactapi_virtual" "alpine" {
+  name         = "alpine"
+  package_type = "alpine"
+  members      = [artifactapi_local_alpine.internal.name, artifactapi_remote_github_alpine.tool.name]
+}
+```
+
+```sh
+echo https://artifactapi.example.com/api/v1/virtual/alpine >> /etc/apk/repositories
+apk update --allow-untrusted
+apk add --allow-untrusted mytool
+```
+
+Upload to the local member directly; the virtual is read-only.