feat: forward Accept header and fix Content-Type for Docker proxying (#62)

## Problems
1. Docker daemon sends specific Accept headers to negotiate manifest format, but the proxy dropped them — registries defaulted to OCI format, causing "mediaType should be manifest.v2+json not oci.image.index" errors
2. Upstream Content-Type was only used when the provider returned "application/octet-stream" — Docker manifests got the wrong Content-Type

## Fixes
- Forward client Accept header to upstream (both initial request and Bearer token retry)
- Always prefer upstream Content-Type when present
- Fetch signature now accepts variadic clientHeaders for backwards compat

## E2E tested
- DockerHub: redis:7-alpine, alpine:3 — skopeo inspect OK
- GHCR: OCI-only images work with docker pull (GHCR 404s Docker v2 Accept, which is expected)
- Quay: prometheus/node-exporter — skopeo inspect OK

Reviewed-on: #62
Co-authored-by: Ben Vincent <ben@unkin.net>
Co-committed-by: Ben Vincent <ben@unkin.net>

internal/api/v1/proxy.go

@@ -67,7 +67,7 @@ func (h *ProxyHandler) handleProxy(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	result, err := h.engine.Fetch(r.Context(), *remote, path, prov)
+	result, err := h.engine.Fetch(r.Context(), *remote, path, prov, r.Header)
 	if err != nil {
 		var proxyErr *proxy.ProxyError
 		if errors.As(err, &proxyErr) {

internal/proxy/engine.go

@@ -44,7 +44,7 @@ type FetchResult struct {
 	Source      string // "cache" or "remote"
 }
 
-func (e *Engine) Fetch(ctx context.Context, remote models.Remote, path string, prov provider.Provider) (*FetchResult, error) {
+func (e *Engine) Fetch(ctx context.Context, remote models.Remote, path string, prov provider.Provider, clientHeaders ...http.Header) (*FetchResult, error) {
 	classifier := NewClassifier(prov)
 	class := classifier.Classify(remote, path)
 
@@ -105,8 +105,13 @@ func (e *Engine) Fetch(ctx context.Context, remote models.Remote, path string, p
 		}
 	}
 
+	var fwdHeaders http.Header
+	if len(clientHeaders) > 0 && clientHeaders[0] != nil {
+		fwdHeaders = clientHeaders[0]
+	}
+
 	start := time.Now()
-	result, err := e.fetchFromUpstream(ctx, remote, path, prov, class, ttl)
+	result, err := e.fetchFromUpstream(ctx, remote, path, prov, class, ttl, fwdHeaders)
 	upstreamMS := int(time.Since(start).Milliseconds())
 	if err != nil {
 		if remote.StaleOnError && isNetworkError(err) {
@@ -126,7 +131,7 @@ func (e *Engine) Fetch(ctx context.Context, remote models.Remote, path string, p
 	return result, nil
 }
 
-func (e *Engine) fetchFromUpstream(ctx context.Context, remote models.Remote, path string, prov provider.Provider, class Classification, ttl time.Duration) (*FetchResult, error) {
+func (e *Engine) fetchFromUpstream(ctx context.Context, remote models.Remote, path string, prov provider.Provider, class Classification, ttl time.Duration, clientHeaders http.Header) (*FetchResult, error) {
 	url := prov.UpstreamURL(remote, path)
 
 	authHeaders, err := prov.AuthHeaders(ctx, remote)
@@ -143,6 +148,11 @@ func (e *Engine) fetchFromUpstream(ctx context.Context, remote models.Remote, pa
 			req.Header.Add(k, v)
 		}
 	}
+	if clientHeaders != nil {
+		if accept := clientHeaders.Get("Accept"); accept != "" {
+			req.Header.Set("Accept", accept)
+		}
+	}
 
 	resp, err := http.DefaultClient.Do(req)
 	if err != nil {
@@ -155,6 +165,11 @@ func (e *Engine) fetchFromUpstream(ctx context.Context, remote models.Remote, pa
 		if err == nil && token != "" {
 			req2, _ := http.NewRequestWithContext(ctx, http.MethodGet, url, nil)
 			req2.Header.Set("Authorization", "Bearer "+token)
+			if clientHeaders != nil {
+				if accept := clientHeaders.Get("Accept"); accept != "" {
+					req2.Header.Set("Accept", accept)
+				}
+			}
 			resp, err = http.DefaultClient.Do(req2)
 			if err != nil {
 				return nil, &UpstreamError{Err: err}
@@ -184,7 +199,7 @@ func (e *Engine) fetchFromUpstream(ctx context.Context, remote models.Remote, pa
 	}
 
 	contentType := prov.ContentType(path)
-	if ct := resp.Header.Get("Content-Type"); ct != "" && contentType == "application/octet-stream" {
+	if ct := resp.Header.Get("Content-Type"); ct != "" {
 		contentType = ct
 	}