Check DOCKER-USER chain exists in nf_tables

[steiler]

The simple check for the loaded nf_tables kernel module is not enough. It can be loaded whilst nf_tables is still not in use. iptables-legacy then most probably is.
So via this PR, we check if the DOCKER-USER chain exists. If not, we return and iptables will be used.

[aminato-nokia]

tested in RHEL 7 with this kernel

[root@kvm-7 tmp]# uname -r
6.7.4-1.el7.elrepo.x86_64

deployment was ok without any error messages

[root@kvm-7 tmp]# containerlab deploy --topo test_ne.yml
INFO[0000] Containerlab v0.49.0 started
INFO[0000] Parsing & checking topology file: test_ne.yml
INFO[0000] Creating lab directory: /tmp/clab-test_oob
INFO[0000] Creating container: "VSR-NRC"
INFO[0000] Adding containerlab host entries to /etc/hosts file
INFO[0000] Adding ssh config for containerlab nodes
+---+-----------------------+--------------+-------------------------------------------+---------+---------+----------------+--------------+
| # |         Name          | Container ID |                   Image                   |  Kind   |  State  |  IPv4 Address  | IPv6 Address |
+---+-----------------------+--------------+-------------------------------------------+---------+---------+----------------+--------------+
| 1 | clab-test_oob-VSR-NRC | 4bb896de89ea | registry.srlinux.dev/pub/vr-sros:23.10.R1 | vr-sros | running | 20.30.40.20/24 | N/A          |
+---+-----------------------+--------------+-------------------------------------------+---------+---------+----------------+--------------+

[steiler]

@hellt Take a look if you've got comments, otherwise we can merge.

[hellt]

Can we check for docker user chain and not just some chains?

[steiler]

Thats what we do. if you look closely you will notice definitions.DockerFWUserChain as the parameter to getChains(...).

[codecov]

Codecov Report

Attention: 5 lines in your changes are missing coverage. Please review.

Comparison is base (aaeb64b) 53.65% compared to head (d7130b3) 53.60%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1900      +/-   ##
==========================================
- Coverage   53.65%   53.60%   -0.05%     
==========================================
  Files         154      154              
  Lines       11227    11233       +6     
==========================================
- Hits         6024     6022       -2     
- Misses       4343     4348       +5     
- Partials      860      863       +3     

Files	Coverage Δ
runtime/docker/firewall/nftables/client.go	70.54% <16.66%> (-2.63%)	⬇️

... and 2 files with indirect coverage changes