Werk #6412: mk_docker_container_piggybacked: don't try to execute the check_mk_agent inside a container if no bash is available

If bash is not available in a docker container docker reports the error on stdout. The mk_docker_container_piggybacked plugin misinterpreted the error as an available check_mk_agent inside of the container.

Affected hosts using the mk_docker_container_piggybacked plugin have to update the plugin to the new version to avoid this behaviour.

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