CLI: Filter Docker containers in a more clever way

[danielledeleo]

See this PR comment: https://github.com/nginx/unit/pull/1241/files#r1594347912

As discussed with @avahahn this can likely be solved by filtering on Docker image labels, one of these, e.g. https://github.com/nginx/unit/blob/5d1ce5c4475f8d126b61650f3c678e3f94549f6b/pkg/docker/Dockerfile.minimal#L3-L9