Werk #15737: docker_node_info: Fix sporadic UNKN if host is monitored by multiple sites

In case a Docker host with installed Docker agent plugin (mk_docker) was monitored by multiple Checkmk sites, the Docker node info service might have sporadically reported UNKN with the following error message:

Plugin exception in section_node_disk_usage: [Errno 2] No such file or directory: '/var/lib/check_mk_agent/mk_docker_df.tmp....' -> '/var/lib/check_mk_agent/mk_docker_df.spool'

To benefit from this werk, users have to re-deploy the Docker agent plugin on affected hosts.

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