Fix Go command PATH resolution in serena container

[Copilot]

Smoke tests reported go: command not found when executing Go commands in the serena MCP server container.

The Dockerfile explicitly added /usr/bin to PATH:

ENV PATH="${GOPATH}/bin:/usr/bin:${PATH}"

This is redundant—/usr/bin is already in the base image's PATH—and interferes with command resolution.

Changes

• Dockerfile: Remove explicit /usr/bin from PATH, append only ${GOPATH}/bin
• BUILD_NOTES.md: Document the fix

# Before
ENV PATH="${GOPATH}/bin:/usr/bin:${PATH}"

# After  
ENV PATH="${GOPATH}/bin:${PATH}"

The simplified configuration ensures both go (from /usr/bin) and gopls (from /go/bin) are accessible without PATH manipulation side effects.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Smoke Copilot - Issue Group</issue_title>
<issue_description># Smoke Copilot

Parent issue for grouping related issues from Smoke Copilot.

Sub-issues are automatically linked below (max 64 per parent).
</issue_description>

<agent_instructions>Check into why the serena container cannot find any go commands in its PATH</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes Smoke Copilot - Issue Group #337

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