add cache-proxy test (#557)

test to verify that a cache stored in one repo can't be restored in another

Reviewed-on: https://code.forgejo.org/forgejo/end-to-end/pulls/557
Reviewed-by: Gusted <gusted@noreply.code.forgejo.org>
Reviewed-by: earl-warren <earl-warren@noreply.code.forgejo.org>
Co-authored-by: Kwonunn <kwonunnx@gmail.com>
Co-committed-by: Kwonunn <kwonunnx@gmail.com>

actions/example-cache-proxy/runner-config.yaml

@@ -0,0 +1,29 @@
+log:
+  level: debug
+
+runner:
+  file: .runner
+  capacity: 1
+  env_file: .env
+  timeout: 3h
+  insecure: false
+  fetch_timeout: 5s
+  fetch_interval: 2s
+  labels: ["docker:docker://code.forgejo.org/oci/node:20-bookworm"]
+
+cache:
+  enabled: true
+  dir: "/srv/example/cache"
+  host: ""
+  port: 0
+
+container:
+  network: "bridge"
+  privileged: false
+  options:
+  workdir_parent:
+  valid_volumes: ["/srv/example"]
+  docker_host: ""
+
+host:
+  workdir_parent: