Move DOCKER_HOST to runner config instead of workflow

The runner's container.env config sets environment variables in all
job containers, so we don't need to specify DOCKER_HOST per-workflow.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

.forgejo/workflows/build-container.yaml

@@ -16,9 +16,6 @@ on:
 jobs:
   build:
     runs-on: k8s
-    env:
-      # For k8s runner with DinD sidecar, Docker is available via TCP
-      DOCKER_HOST: tcp://127.0.0.1:2375
     steps:
       - name: Parse tag
         id: parse

argocd/manifests/forgejo-runner/configmap.yaml

@@ -18,3 +18,6 @@ data:
       network: "host"
       # Connect to DinD sidecar via TCP (not socket)
       docker_host: tcp://127.0.0.1:2375
+      # Set DOCKER_HOST in job containers so they can run docker commands
+      env:
+        DOCKER_HOST: tcp://127.0.0.1:2375