feat(agent): send the CPU count and the Docker state in the status

[nabil1440]

Closes #44.

Part of the monitoring agent stack. This PR is on top of #48. Contract v0.5.0, "status: the CPU count and Docker".

What changes

• status.cpu_count: the number of cpuN lines in /proc/stat.
• New package internal/dockerapi. A client for the Docker Engine API on the unix socket, with the Go standard library only. It sends only GET /version and GET /containers/json, with a limit of 5 seconds for each request. It does not use the docker command.
• status.docker_status, in the order of the contract:
  • running when GET /version answers
  • not_running when there is no socket, or the connection is refused, and /usr/bin/dockerd exists
  • not_installed in the same case without /usr/bin/dockerd
  • null for each other result, for example "permission denied" or no answer in 5 seconds. The agent writes one warning.
• status.docker_version: the Version of the answer, only when docker_status is running.
• The agent keeps each value in its range: cpu_count 1 to 4096, a known docker_status, and a docker_version of at most 32 characters.

Caution

• Access to the Docker socket is root access. The agent user already has it on a FlyWP server. The agent sends only the two requests above, and a test examines this.

Tests

• A fake Docker Engine on a unix socket: running, no socket with and without dockerd, a refused connection, a socket that refuses the user, and no answer.
• make check passes.