fix(ci): drop volumes block — act runner already mounts docker.sock

act (the runner the user is running) auto-mounts the docker
socket via volume `GITEA-ACTIONS-TASK-...-env Target:/var/run/act`.
Declaring volumes: in the workflow causes a duplicate mount
(once via the volumes block, once via act's auto-mount):

  Binds:[/var/run/docker.sock:/var/run/docker.sock
         /var/run/docker.sock:/var/run/docker.sock]

  failed to create container:
    'Error response from daemon: Duplicate mount point'

Drop the explicit volumes block. The socket is already there.
The docker CLI inside the container can talk to the host daemon
via the already-mounted socket.

.gitea/workflows/ci.yml

@@ -20,8 +20,6 @@ jobs:
     container:
       image: maven:3.9.6-eclipse-temurin-21
       options: --memory=8g
-      volumes:
-        - /var/run/docker.sock:/var/run/docker.sock
     steps:
       - name: Install git, node, gcc, docker, and basic tools
         run: |