Dockerfile: use /var/lib/etcd as default data dir

[gyuho]

No description provided.

[gyuho]

/cc @xiang90

[xiang90]

well... shall we just revert that pr then? what are the other stuff we have changed?

[gyuho]

@xiang90 Yeah let's revert it. Here #5937.

And I think we should use /var/lib/etcd too, following this example? https://github.com/coreos/etcd/blob/master/contrib/systemd/etcd.service

[xiang90]

what was it before? i do not want to change it if it was /var/etcd. it can break users after they use new image.

[gyuho]

@xiang90 There was no default data dir before. It just mkdir -p /var/etcd.

If people run etcd in docker before without any flag, it will default to default.data.

[xiang90]

If you change the default data dir, then it breaks existing users. after they restart etcd with new image, they cannot find the old data-dir since the default location changed. no?

[gyuho]

Yeah you are right. Let's keep it as it is now.

Thanks!