Add Alpine/apk local repository support

The alpine provider hosted only remote (proxy) repos; there was no way to
publish first-party .apk packages the way rpm-local and deb-local already
allow. This extends the existing alpine provider into a real apk repository:
uploaded .apk files are parsed in pure Go and a per-arch APKINDEX.tar.gz is
generated on demand, at parity with rpm repodata and deb Packages generation.

- Implement LocalUploader/LocalIndexer/PostUploadHook/PostDeleteHook on the
  alpine provider, keeping the remote proxy methods intact.
- Parse .apk (concatenated gzipped tar streams) in pure Go: read .PKGINFO from
  the control stream and compute the apk pull checksum C: = Q1+base64(sha1) over
  the raw control gzip stream (not the whole file).
- Generate an unsigned per-arch APKINDEX.tar.gz (clients use --allow-untrusted),
  applying the same dot-segment normalization as deb for ./<arch>/... requests.
- Add AlpineMetadata plus separate Alpine store/reader/deleter interfaces so the
  rpm/deb metadata interfaces are not widened.
- Add the alpine_metadata table and its Insert/Delete/List DB methods.
- Add testsupport.MinimalApk plus unit tests (parse, Q1 checksum, per-arch
  filtering, dot-segment handling, validate) and a dockere2e index test.

internal/provider/provider.go

@@ -115,6 +115,50 @@ type DebMetadata struct {
 	SHA256       string
 }
 
+// AlpineMetadataStore / AlpineMetadataDeleter / AlpineMetadataReader are the
+// Alpine-specific persistence surfaces. They are kept separate from the shared
+// RPM/Deb metadata interfaces so the apk provider can type-assert the generic
+// MetadataStore/MetadataDeleter/FileStore it is handed without widening (and
+// thus perturbing the test doubles of) the rpm and deb providers. *database.DB
+// satisfies all three.
+type AlpineMetadataStore interface {
+	InsertAlpineMetadata(ctx context.Context, meta *AlpineMetadata) error
+}
+
+type AlpineMetadataDeleter interface {
+	DeleteAlpineMetadata(ctx context.Context, repoName, filePath string) error
+}
+
+type AlpineMetadataReader interface {
+	ListAlpineMetadataEntries(ctx context.Context, repoName string) ([]AlpineMetadata, error)
+}
+
+// AlpineMetadata is the derived per-package metadata for an Alpine .apk, holding
+// the fields an APKINDEX record carries plus the apk pull checksum (Q1…, the
+// sha1 of the control gzip stream) and the download/installed sizes.
+type AlpineMetadata struct {
+	RepoName         string
+	FilePath         string
+	ContentHash      string
+	Checksum         string   // C: "Q1" + base64(sha1(control gzip stream))
+	Name             string   // P:
+	Version          string   // V:
+	Arch             string   // A:
+	DownloadSize     int64    // S: on-disk .apk size
+	InstalledSize    int64    // I: unpacked size from .PKGINFO
+	Description      string   // T:
+	URL              string   // U:
+	License          string   // L:
+	Origin           string   // o:
+	Maintainer       string   // m:
+	BuildTime        int64    // t:
+	Commit           string   // c:
+	ProviderPriority string   // k:
+	Depends          []string // D:
+	Provides         []string // p:
+	InstallIf        []string // i:
+}
+
 type RPMMetadata struct {
 	RepoName      string
 	FilePath      string