test: add comprehensive dockerised end-to-end suite

Add a black-box e2e suite (build tag dockere2e) that runs against the
built container image via docker-compose, plus a static nginx mock
upstream for hermetic caching tests.

Coverage:
- repository add/change/delete for remote, local and virtual repos
- caching (miss -> hit + byte integrity) for all 10 remote package types
- local uploads: generic, pypi (with generated simple index), rpm (with
  automatic repodata generation from a real package)
- virtual merges: pypi simple index and helm index.yaml

Driven by scripts/docker-e2e.sh (make docker-e2e): builds the image,
brings the stack up, waits for health, runs the suite, and tears down.
The artifactapi host port is parameterised (ARTIFACTAPI_PORT, default
8000; the e2e run uses 8001). Fixtures are force-tracked over the global
gitignore.

e2e-docker/caching_test.go

@@ -0,0 +1,76 @@
+//go:build dockere2e
+
+package e2edocker
+
+import (
+	"bytes"
+	"fmt"
+	"net/http"
+	"testing"
+)
+
+// TestCachingPerProvider proxies one immutable artifact for every remote
+// package type through the mock upstream and asserts: first fetch is served
+// from the remote, the second from cache, and the bytes match the origin.
+func TestCachingPerProvider(t *testing.T) {
+	cases := []struct {
+		pkgType string
+		// path is the request path under /api/v1/remote/<name>/. The provider
+		// derives the upstream URL from it (docker prepends /v2/, terraform
+		// prepends /v1/providers/), and the fixture lives at that resolved path.
+		path    string
+		fixture string
+	}{
+		{"generic", "blobs/hello.bin", "blobs/hello.bin"},
+		{"npm", "mypkg/-/mypkg-1.0.0.tgz", "mypkg/-/mypkg-1.0.0.tgz"},
+		{"helm", "charts/mychart-1.0.0.tgz", "charts/mychart-1.0.0.tgz"},
+		{"pypi", "packages/foo-1.0-py3-none-any.whl", "packages/foo-1.0-py3-none-any.whl"},
+		{"rpm", "rpmrepo/Packages/e2e-testpkg-1.0-1.noarch.rpm", "rpmrepo/Packages/e2e-testpkg-1.0-1.noarch.rpm"},
+		{"alpine", "alpine/x86_64/testpkg-1.0-r0.apk", "alpine/x86_64/testpkg-1.0-r0.apk"},
+		{"puppet", "puppet-releases/author-mod-1.0.0.tar.gz", "puppet-releases/author-mod-1.0.0.tar.gz"},
+		{"goproxy", "goproxy/example.com/mod/@v/v1.0.0.zip", "goproxy/example.com/mod/@v/v1.0.0.zip"},
+		{"terraform", "hashicorp/aws/download/pkg.zip", "v1/providers/hashicorp/aws/download/pkg.zip"},
+		{"docker", "library/testimg/blobs/blobdata", "v2/library/testimg/blobs/blobdata"},
+	}
+
+	for _, tc := range cases {
+		t.Run(tc.pkgType, func(t *testing.T) {
+			name := "cache-" + tc.pkgType
+			createRepo(t, fmt.Sprintf(`{
+				"name": %q,
+				"package_type": %q,
+				"repo_type": "remote",
+				"base_url": %q,
+				"stale_on_error": true
+			}`, name, tc.pkgType, mockUpstream()))
+			defer deleteRepo(t, name)
+
+			want := fixtureBytes(t, tc.fixture)
+			url := api("/api/v1/remote/" + name + "/" + tc.path)
+
+			// First fetch: from remote.
+			resp, body := doRequest(t, http.MethodGet, url, nil, "")
+			if resp.StatusCode != http.StatusOK {
+				t.Fatalf("first fetch: status %d: %s", resp.StatusCode, body)
+			}
+			if src := resp.Header.Get("X-Artifact-Source"); src != "remote" {
+				t.Fatalf("first fetch source = %q, want remote", src)
+			}
+			if !bytes.Equal(body, want) {
+				t.Fatalf("first fetch body mismatch: got %d bytes, want %d", len(body), len(want))
+			}
+
+			// Second fetch: from cache, identical bytes.
+			resp, body = doRequest(t, http.MethodGet, url, nil, "")
+			if resp.StatusCode != http.StatusOK {
+				t.Fatalf("second fetch: status %d: %s", resp.StatusCode, body)
+			}
+			if src := resp.Header.Get("X-Artifact-Source"); src != "cache" {
+				t.Fatalf("second fetch source = %q, want cache", src)
+			}
+			if !bytes.Equal(body, want) {
+				t.Fatalf("cached body mismatch: got %d bytes, want %d", len(body), len(want))
+			}
+		})
+	}
+}