[nabil1440]

Change

• docker.Check examines three parts, each with a 5 s timeout:
  • the Docker CLI (PATH)
  • the Compose plugin (docker compose version --short)
  • the daemon (docker version --format {{.Server.Version}})
• Commands that need Docker declare it with requireDocker. The root command runs the check before these commands.
• If a part is not available, fly shows one warning line and exits with status 69 (EX_UNAVAILABLE). No command panics, and no raw exec: error shows.
• fly status shows the three parts on separate lines and exits with status 0.

Why this design

• requireDocker marks the commands that need Docker. The capability work can use the same mark to set the Docker capability online or offline.
• When Docker is healthy, the check adds approximately 30 ms.

Tests

• Unit tests for Check: available, no CLI, no Compose plugin, daemon down and daemon timeout. Unit tests for Status.
• End-to-end tests: 5 commands in 3 failure conditions give exit status 69, one warning line and no Compose call. fly status gives exit status 0 and three lines.
• Real Docker:
  • When Docker is healthy, fly status shows the three parts.
  • With DOCKER_HOST set to a socket that does not exist, fly start shows the daemon warning and exits with status 69.
  • With no docker in PATH, fly wp shows the CLI warning and exits with status 69.

Closes #5