test: comprehensive dockerised end-to-end suite (#97)

Adds a black-box e2e suite that runs against the **built container image** via docker-compose (complementing the in-process `e2e/` testcontainers suite).

## What it does
`make docker-e2e` → `scripts/docker-e2e.sh`: builds the image, brings up the full stack (postgres, redis, minio, artifactapi) plus a static nginx **mock upstream** for hermetic caching, waits for `/health`, runs `go test -tags=dockere2e ./e2e-docker/...`, and tears everything down.

## Coverage
- **Repository lifecycle** — add / change / delete for remote, local and virtual repos.
- **Caching** — one immutable artifact for **each of the 10 remote package types** (generic, docker, helm, pypi, npm, rpm, alpine, puppet, terraform, goproxy) proxied through the mock upstream: first fetch `X-Artifact-Source: remote`, second `cache`, bytes verified against the origin fixture.
- **Local uploads** — generic (upload/download), pypi (wheel + generated `simple/` index), rpm (real package + **automatic repodata** generation).
- **Virtual repositories** — pypi simple-index merge and helm `index.yaml` merge across two members.

## Notes
- The artifactapi host port is parameterised (`ARTIFACTAPI_PORT`, default `8000`; the e2e run uses `8001`) so it does not collide with a locally-running instance. This is the only change to the production `docker-compose.yml`.
- Fixtures under `e2e-docker/fixtures/` are real package files (incl. a real RPM so repodata parsing works); a `.gitignore` negation tracks them over the global ignore of those extensions.

## Validation
Ran `make docker-e2e` locally: **all suites pass** against the containerised product.

---------

Co-authored-by: BenVincent <benvin@main.unkin.net>
Reviewed-on: #97
Co-authored-by: Ben Vincent <ben@unkin.net>
Co-committed-by: Ben Vincent <ben@unkin.net>

docker-compose.e2e.yml

@@ -0,0 +1,18 @@
+# Overlay for the dockerised end-to-end suite (scripts/docker-e2e.sh).
+# Adds a static mock upstream that the artifactapi container proxies, so the
+# caching tests are hermetic and need no internet access.
+services:
+  mockupstream:
+    image: nginx:alpine
+    volumes:
+      - ./e2e-docker/fixtures:/usr/share/nginx/html:ro,z
+    # No host port needed: only the artifactapi container talks to it, and the
+    # tests compare served bytes against the on-disk fixtures.
+
+  artifactapi:
+    # The host port is set via ARTIFACTAPI_PORT (see scripts/docker-e2e.sh),
+    # defaulting to 8000; the e2e run uses 8001 to avoid colliding with a
+    # locally-running instance.
+    depends_on:
+      mockupstream:
+        condition: service_started