fix: use container DNS name endpoint, executor runs on same docker network

Root cause of the 'waiting for connection' build failure: the act_runner
executor container runs on the same docker network as buildkitd, so its
127.0.0.1 is its own loopback — the host-published port is unreachable
from there. Revert to tcp://<name>:<port>; container DNS resolves on the
shared network. Drop the now-useless loopback port publish.

Verified on host: from a container on that network, endpoint
tcp://<name>:8375 -> 'Status: running'; tcp://127.0.0.1:8375 ->
'ERROR: context deadline exceeded' (exact CI symptom).

action.yml

@@ -41,9 +41,11 @@ runs:
         # If it is already running with the right config, keep it (warm cache).
         if [ "$(docker inspect -f '{{.State.Running}}' ${{ inputs.container_name }} 2>/dev/null)" != "true" ]; then
           docker rm -f ${{ inputs.container_name }} >/dev/null 2>&1 || true
+          # NOTE: no -p publish needed — the job executor itself runs on the
+          # same docker network, so it reaches buildkitd by container DNS name.
+          # (127.0.0.1 inside the executor is its own loopback, NOT the host's.)
           docker run -d --name ${{ inputs.container_name }} --privileged \
             --network ${{ inputs.network }} \
-            -p 127.0.0.1:${{ inputs.port }}:${{ inputs.port }} \
             --restart unless-stopped \
             --label com.centurylinklabs.watchtower.monitor-only=${{ inputs.monitor_only }} \
             --label dockhand.notify=${{ inputs.dockhand_notify }} \
@@ -54,7 +56,7 @@ runs:
       uses: docker/setup-buildx-action@v4
       with:
         driver: remote
-        endpoint: tcp://127.0.0.1:${{ inputs.port }}
+        endpoint: tcp://${{ inputs.container_name }}:${{ inputs.port }}
 
     - name: Stop BuildKit (idle cleanup)
       if: ${{ inputs.cleanup == 'true' && always() }}