Ensure that hostname is added to hosts with net=host

[mheon]

When a container uses --net=host the default hostname is set to the host's hostname. However, we were not creating any entries in /etc/hosts despite having a hostname, which is incorrect.

Fixes #8054

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[zhangguanzhang]

seem docker don't have this?

[root@k8s-m1 ~]# docker run --hostname test111  --net host nginx:alpine cat /etc/hosts | grep test111
[root@k8s-m1 ~]# docker info
Client:
 Debug Mode: false

Server:
 Containers: 13
  Running: 9
  Paused: 0
  Stopped: 4
 Images: 22
 Server Version: 19.03.2
 Storage Driver: overlay2
  Backing Filesystem: extfs
  Supports d_type: true
  Native Overlay Diff: true
 Logging Driver: json-file
 Cgroup Driver: systemd
 Plugins:
  Volume: local
  Network: bridge host ipvlan macvlan null overlay
  Log: awslogs fluentd gcplogs gelf journald json-file local logentries splunk syslog
 Swarm: inactive
 Runtimes: runc
 Default Runtime: runc
 Init Binary: docker-init
 containerd version: b34a5c8af56e510852c35414db4c1f4fa6172339
 runc version: 3e425f80a8c931f88e6d94a8c831b9d5aa481657
 init version: fec3683
 Security Options:
  seccomp
   Profile: default
 Kernel Version: 5.8.9-1.el7.elrepo.x86_64
 Operating System: CentOS Linux 7 (Core)
 OSType: linux
 Architecture: x86_64
 CPUs: 1
 Total Memory: 1.943GiB
 Name: k8s-m1
 ID: EIUG:BTLZ:UK4M:P6C5:L2IW:UXEA:ZTL5:6CHJ:I3CL:G2UM:SGQ5:LG6S
 Docker Root Dir: /var/lib/docker
 Debug Mode: false
 Username: zhangguanzhang
 Registry: https://index.docker.io/v1/
 Labels:
 Experimental: false
 Insecure Registries:
  127.0.0.0/8
 Registry Mirrors:
  https://fz5yth0r.mirror.aliyuncs.com/
 Live Restore Enabled: false

[mheon]

Docker is unconditionally forcing the entry in /etc/hosts to use the hostname of the host system, not the container. I'll adjust to match this.

[mheon]

New changes: we now respect --hostname if passed, even if --net=host was given, and the hostname added to /etc/hosts when --net=host is set will always be the host system's hostname, not the container's hostname - both match Docker.

[TomSweeneyRedHat]

code LGTM
any notes/changes to man pages needed?

[rhatdan]

Shouldn't --net=host and --hostname conflict?
I don't believe a container should be able to change the hosts namespace, unless it has NET_ADMIN capability?

[mheon]

@rhatdan I don't think we are changing the host, since the container does have a UTS namespace in this case - it's just missing the net namespace.

[rhatdan]

Ah OK, I guess they should conflict with the --uts=host, but no one ever does this.

[rhatdan]

/lgtm