Use an empty slice as default value for DNS, DNSSearch and DNSOptions
Use an empty slice as default value for DNS, DNSSearch and DNSOptions

So we don't print those <no value> in the client and we don't fail
executing inspect templates with API field names.

Make sure those fields are initialized as empty slices when
a container is loaded from disk and their values are nil.

Signed-off-by: David Calavera <b9b0c8e7f90ecf337f6797c4d2e0f1dd0dbc0324@gmail.com>

docker/cli,allencloud/cli,e11137/cli,Zebrilee/cli,cpuguy83/cli,riyazdf/cli,e11137/cli,allencloud/cli,thaJeztah/cli,redpanda/cli,redpanda/cli,dnephin/cli,docker/cli,cpuguy83/cli,ashfall/cli,Zebrilee/cli,thaJeztah/cli,dnephin/cli,riyazdf/cli,ashfall/cli