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/fixtures/blobs/hello.bin

@@ -0,0 +1 @@
+hello artifactapi generic blob

e2e-docker/fixtures/helm-a/index.yaml

@@ -0,0 +1,8 @@
+apiVersion: v1
+entries:
+  alpha:
+    - name: alpha
+      version: 1.0.0
+      urls:
+        - charts/alpha-1.0.0.tgz
+generated: "2026-01-01T00:00:00Z"

e2e-docker/fixtures/helm-b/index.yaml

@@ -0,0 +1,8 @@
+apiVersion: v1
+entries:
+  beta:
+    - name: beta
+      version: 2.0.0
+      urls:
+        - charts/beta-2.0.0.tgz
+generated: "2026-01-01T00:00:00Z"