Add MCP Gateway build steps to language-support-tester workflow (#1089)

The Language Support Tester workflow failed because it referenced the
registry image `ghcr.io/github/gh-aw-mcpg` without building it locally
first, causing Docker daemon connectivity issues and missing Serena MCP
tools.

## Changes

Added local container build steps following the `smoke-copilot.md`
pattern:

- **Docker Buildx setup** - Added `docker/setup-buildx-action@v3` step
- **Local MCP Gateway build** - Builds `local-awmg:v0.1.4` with embedded
`awmg` binary
- **Container reference** - Updated `sandbox.mcp.container` from
`ghcr.io/github/gh-aw-mcpg` to `local-awmg`

```yaml
steps:
  - name: Set up Docker Buildx
    uses: docker/setup-buildx-action@v3
  - name: Build local MCP Gateway container
    run: |
      VERSION="dev-$(git rev-parse --short HEAD)"
      docker build -t local-awmg:v0.1.4 --build-arg VERSION=${VERSION} .

sandbox:
  mcp:
    container: "local-awmg"  # Changed from registry reference
```

This ensures the MCP Gateway is available without external registry
dependencies, enabling Serena tools (`activate_project`, `find_symbols`,
etc.) for Go, TypeScript/JavaScript, and Python language testing.

> [!WARNING]
>
> <details>
> <summary>Firewall rules blocked me from connecting to one or more
addresses (expand for details)</summary>
>
> #### I tried to connect to the following addresses, but was blocked by
firewall rules:
>
> -
`https://api.github.com/repos/docker/setup-buildx-action/git/ref/tags/v3`
> - Triggering command: `/usr/bin/gh gh api
/repos/docker/setup-buildx-action/git/ref/tags/v3 --jq .object.sha`
(http block)
> - `https://api.github.com/repos/github/gh-aw/git/ref/tags/v0.46.1`
> - Triggering command: `/usr/bin/gh gh api
/repos/github/gh-aw/git/ref/tags/v0.46.1 --jq .object.sha` (http block)
> - `https://api.github.com/user`
> - Triggering command: `/usr/bin/gh gh api user --jq .login
-main/dist/indexuser.name` (http block)
>
> If you need me to access, download, or install something from one of
these locations, you can either:
>
> - Configure [Actions setup
steps](https://gh.io/copilot/actions-setup-steps) to set up my
environment, which run before the firewall is enabled
> - Add the appropriate URLs or hosts to the custom allowlist in this
repository's [Copilot coding agent
settings](https://github.com/github/gh-aw-mcpg/settings/copilot/coding_agent)
(admins only)
>
> </details>

<!-- START COPILOT ORIGINAL PROMPT -->



<details>

<summary>Original prompt</summary>

> 
> ----
> 
> *This section details on the original issue you should resolve*
> 
> <issue_title>[language-support] Serena MCP server tools unavailable —
language support tests could not run</issue_title>
> <issue_description>## Summary
> 
> The Language Support Tester workflow (run #34, run ID
22164360544) could not complete language support testing for **Go**,
**TypeScript/JavaScript**, and **Python** because the Serena MCP server
tools were not available in the sandbox environment.
> 
> ## Failure Details
> 
> ### Root Cause
> 
> The Docker daemon is not accessible from within the sandbox container:
> 
> ```
> Cannot connect to the Docker daemon at unix:///var/run/docker.sock. Is
the docker daemon running?
> ```
> 
> Because Docker is unavailable, the Serena MCP server container
(`ghcr.io/github/serena-mcp-server:latest`) cannot be started. As a
result, none of the language server tools (`activate_project`,
`find_symbols`, `get_definition`, etc.) were present in the agent's
available tool list.
> 
> ### Diagnostic evidence
> 
> - `/tmp/gh-aw/mcp-logs/` — empty (no MCP server logs, no `tools.json`)
> - `/tmp/gh-aw/mcp-config/` — contains only `jq-error.log`, no MCP
configuration JSON files
> - `docker images` / `docker ps` — command fails: daemon not reachable
> - `awmg` binary — not found in `PATH`
> - Serena tools (`activate_project`, `find_symbols`, etc.) — absent
from agent function-call interface
> 
> ### Languages affected
> 
> | Language | Status |
> |---|---|
> | Go | ❌ Not tested — Serena unavailable |
> | TypeScript/JavaScript | ❌ Not tested — Serena unavailable |
> | Python | ❌ Not tested — Serena unavailable |
> 
> ## Steps to Reproduce
> 
> 1. Trigger the **Language Support Tester** workflow
> 2. Observe that no Serena MCP tools appear in the agent's available
tools
> 3. `docker ps` inside the sandbox returns a daemon connection error
> 
> ## Expected Behavior
> 
> - Serena MCP server starts successfully
> - `activate_project`, `find_symbols`, `get_definition` tools are
available
> - Language support is verified for Go, TypeScript/JavaScript, and
Python
> 
> ## Suggested Investigation
> 
> - Verify that the sandbox container has access to the Docker daemon
(Docker-in-Docker or DooD)
> - Confirm the workflow's `tools: serena:` configuration is correctly
translating into MCP server setup
> - Check whether the pre-pull step (`docker pull
ghcr.io/github/serena-mcp-server:latest`) is succeeding before the agent
starts
> 
> 
> 
> 
> > Generated by [Language Support
Tester](https://github.com/github/gh-aw-mcpg/actions/runs/22164360544)
> > - [x] expires <!-- gh-aw-expires: 2026-02-26T01:14:02.300Z --> on
Feb 26, 2026, 1:14 AM UTC
> 
> <!-- gh-aw-agentic-workflow: Language Support Tester, engine: copilot,
run: https://github.com/github/gh-aw-mcpg/actions/runs/22164360544 -->
> 
> <!-- gh-aw-workflow-id: language-support-tester
--></issue_description>
> 
> ## Comments on the Issue (you are @copilot in this section)
> 
> <comments>
> </comments>
> 


</details>



<!-- START COPILOT CODING AGENT SUFFIX -->

- Fixes #1088

<!-- START COPILOT CODING AGENT TIPS -->
---

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Author: lpcox

.github/workflows/language-support-tester.lock.yml

@@ -17,6 +17,13 @@ steps:
     with:
       go-version-file: go.mod
       cache: true
+  - name: Set up Docker Buildx
+    uses: docker/setup-buildx-action@v3
+  - name: Build local MCP Gateway container
+    run: |
+      VERSION="dev-$(git rev-parse --short HEAD)"
+      docker build -t local-awmg:v0.1.4 --build-arg VERSION=${VERSION} .
+      echo "✅ Built local MCP Gateway container: local-awmg:v0.1.4 (VERSION=${VERSION})"
   - name: Pull Serena MCP Server Container
     run: docker pull ghcr.io/github/serena-mcp-server:latest
 tools:
@@ -26,7 +33,7 @@ tools:
 
 sandbox:
   mcp:
-    container: "ghcr.io/github/gh-aw-mcpg"
+    container: "local-awmg"
 
 safe-outputs:
   create-issue: