Add a check to ensure sandbox images are accessable before executing them.

[calebbrown]

The worker/analysis engine should ensure the sandbox images are all available at the start of execution.

This will help catch failures where a sandbox container image is unavailable, or if the environment has been configured incorrectly - and it will help prevent the pubsub messages from being consumed errantly.

[calebbrown]

This should be achievable by using https://pkg.go.dev/github.com/google/go-containerregistry/pkg/v1/remote#Get to check if the image is accessible.