fix(ci-runner): install docker compose v2 plugin correctly

The docker.io apt package doesn't ship the compose plugin, and the
npm docker-compose package is an unrelated legacy tool that doesn't
provide the 'docker compose' CLI subcommand. Install the official
compose v2 binary as a docker CLI plugin instead.

.gitea/runners/Dockerfile

@@ -1,9 +1,15 @@
 FROM node:22-bullseye
 
 RUN apt-get update \
-    && apt-get install -y docker.io \
+    && apt-get install -y docker.io curl \
     && rm -rf /var/lib/apt/lists/*
 
 RUN corepack enable
 
-RUN npm install -g docker-compose
+# Install Docker Compose V2 as a CLI plugin (docker.io package does not
+# include it, and the npm "docker-compose" package is an unrelated
+# legacy wrapper that does not provide the `docker compose` subcommand).
+RUN mkdir -p /usr/libexec/docker/cli-plugins \
+    && curl -SL "https://github.com/docker/compose/releases/latest/download/docker-compose-linux-x86_64" \
+       -o /usr/libexec/docker/cli-plugins/docker-compose \
+    && chmod +x /usr/libexec/docker/cli-plugins/docker-compose