Populate response message

[alexashley]

The enforcer should now return an message when an image fails policy:

Error from server: admission webhook "k8s-enforcer.rode.io" denied the request: container image "harbor.localhost/rode-demo/nginx:latest" failed the Rode policy "Sample Harbor Policy" (id: 38e62c17-750a-43e3-ad54-335571d340c4)

or when an error occurs:

Error from server: admission webhook "k8s-enforcer.rode.io" denied the request: error evaluating policy: rpc error: code = Unavailable desc = connection closed

Also includes some smaller changes:

• new flags:
  • rode-insecure for connecting to Rode without transport security
  • debug flag to configure log level
  • registry-insecure-skip-verify to control whether TLS is verified when the enforcer fetches image manifests from a registry
• refactor server TLS config to avoid writing cert and key to disk
• check init containers against policy (closes #7)
• add source label to Docker image
• handle images from Docker Hub that don't include registry url or are the default library namespace/organization

This will require some changes to the Helm chart to allow for configuring the new flags, which I can do after this is merged and released.

[codecov-commenter]

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