When image digest is included in the container definition image, validation fails

[kevinross]

this sanitizes the colon but excludes the @, so an image of the form some.domain.example/image:version@SHA256:digest is sanitized to image-version@sha256-digest, which causes the agent to reject the log group name with InvalidParameterException: 1 validation error detected: Value 'image-version@sha256-digest-metrics' at 'logGroupName' failed to satisfy constraint: Member must satisfy regular expression pattern: [\\.\\-_/#A-Za-z0-9]+",