feat: add Docker environment diagnostic

[dDevAhmed]

Summary

Implements CLI-021 by adding an optional Docker diagnostic to stellarforge doctor.

Included

• detects the Docker CLI with docker --version
• checks daemon availability with docker info --format '{{.ServerVersion}}'
• distinguishes missing CLI from unavailable daemon
• treats Docker as optional by default for the generic doctor run
• supports required: true so future workflows/templates can escalate missing Docker to a fatal diagnostic
• integrates Docker into the default doctor check set
• adds unit coverage for present, missing, required, daemon-unavailable, malformed, and secret-safe failure paths
• updates the compiled executable doctor test to include Docker without assuming runner-specific availability

Security

• no shell interpolation
• no privileged Docker operations
• no container execution
• no Docker configuration/environment dumping
• subprocess stderr/errors are not rendered into user-facing diagnostics

Closes #24