• backport: Add escape hatch for GODEBUG=x509negativeserial #6371

- What I did
Negative serial number certificates are deprecated and Go has also since deprecated them. To use them with Go you need to pass in the GODEBUG=x509negativeserial=1 environment variable.

This PR allows setting the value inside docker context instead. This should then propagate environment variables to underlying plugins (such as buildx).

{
  "Name": "my-context",
  "Metadata": { "GODEBUG": "x509negativeserial=1" },
  "Endpoints": {
    "docker": { "Host": "unix:///var/run/docker.sock", "SkipTLSVerify": false }
  }
}

- How I did it

- How to verify it

- Human readable description for the release notes

Allow Docker CLI to set the `GODEBUG` environment variable when the key-value pair (`"GODEBUG":"..."`) exists inside the docker context metadata.

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