Add a container configuration representation.

[coollog]

Usable representation as well as JSON serialization representation (compatible with #30).

Example json:

{
  "architecture": "amd64",
  "os": "linux",
  "config": {
    "Env": [],
    "Entrypoint": []
  },
  "rootfs": {
    "diff_ids": [
      "sha256:2aebd096e0e237b447781353379722157e6c2d434b9ec5a0d63f2a6f07cf90c2",
      "sha256:5f70bf18a086007016e948b04aed3b82103a36bea41755b6cddfaf10ace3c6ef",
      "sha256:ced237d1396248eab64683aad2f6603874f8a75505b7a48a4c0d9cf260d1d3eb",
      "sha256:97bb5452e0c7d815008f1da7befeec29811ec13ba0c3828442bb2d1ad6566011",
      "sha256:54058933e0ac82e2800c935cf910aa9a190b8703682cdf73012384fd7359fff9",
      "sha256:f4b6aab6e47091e21c6b2fa0939f0176597fc1955e8aab038a2ecff9a85e13ba",
      "sha256:c667a06127bf2b2569603fd8cd255d998b9f383b7254c7d1f688a3d44933dd71"
    ],
    "type": "layers"
  }
}