Container build type

[ianlewis]

Fixes #638

Updates provenance generated by the generic container worflow to have it's own buildType.

Before:

{
  "predicate": {
    "buildType": "https://github.com/slsa-framework/slsa-github-generator/generic@v1"
  }
}

After:

{
  "predicate": {
    "buildType": "https://github.com/slsa-framework/slsa-github-generator/container@v1"
  }
}

This adds a new builder binary. Currently it has mostly the same logic as the generic builder's generate command had but may require more specialized logic in the future.