Flaky test: TestUserChain (libnetwork)

[thaJeztah]

Seen failing on #42445
https://ci-next.docker.com/public/job/moby/job/PR-42445/11/execution/node/333/log/

=== RUN   TestUserChain/iptables=true,insert=true
    --- FAIL: TestUserChain/iptables=true,insert=true (0.00s)
        firewall_linux_test.go:62: assertion failed:
            --- ←
            +++ →
            {[]string}[1->?]:
            	-: "-A FORWARD -i docker0 ! -o docker0 -j ACCEPT"
            	+: <non-existent>
            {[]string}[2->?]:
            	-: "-A FORWARD -i docker0 -o docker0 -j DROP"
            	+: <non-existent>

docker version and info

 Client: Docker Engine - Community
  Version:           20.10.6
  API version:       1.41
  Go version:        go1.13.15
  Git commit:        370c289
  Built:             Fri Apr  9 22:46:01 2021
  OS/Arch:           linux/amd64
  Context:           default
  Experimental:      true

 Server: Docker Engine - Community
  Engine:
   Version:          20.10.6
   API version:      1.41 (minimum version 1.12)
   Go version:       go1.13.15
   Git commit:       8728dd2
   Built:            Fri Apr  9 22:44:13 2021
   OS/Arch:          linux/amd64
   Experimental:     true
  containerd:
   Version:          1.4.6
   GitCommit:        d71fcd7d8303cbf684402823e425e9dd2e99285d
  runc:
   Version:          1.0.0-rc95
   GitCommit:        b9ee9c6314599f1b4a7f497e1f1f856fe433d3b7
  docker-init:
   Version:          0.19.0
   GitCommit:        de40ad0

Client:
  Context:    default
  Debug Mode: false
  Plugins:
   app: Docker App (Docker Inc., v0.9.1-beta3)
   buildx: Build with BuildKit (Docker Inc., v0.5.1-docker)
   scan: Docker Scan (Docker Inc., v0.7.0)

 Server:
  Containers: 0
   Running: 0
   Paused: 0
   Stopped: 0
  Images: 1
  Server Version: 20.10.6
  Storage Driver: overlay2
   Backing Filesystem: extfs
   Supports d_type: true
   Native Overlay Diff: true
   userxattr: false
  Logging Driver: json-file
  Cgroup Driver: cgroupfs
  Cgroup Version: 1
  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: io.containerd.runc.v2 io.containerd.runtime.v1.linux runc
  Default Runtime: runc
  Init Binary: docker-init
  containerd version: d71fcd7d8303cbf684402823e425e9dd2e99285d
  runc version: b9ee9c6314599f1b4a7f497e1f1f856fe433d3b7
  init version: de40ad0
  Security Options:
   apparmor
   seccomp
    Profile: default
  Kernel Version: 5.4.0-1048-aws
  Operating System: Ubuntu 18.04.5 LTS
  OSType: linux
  Architecture: x86_64
  CPUs: 8
  Total Memory: 30.58GiB
  Name: ip-10-100-120-167
  ID: KG52:ONVY:U5MS:VBR3:3V2I:UMRB:NEG5:YUDM:PLPH:2QKQ:N3BG:QG3A
  Docker Root Dir: /var/lib/docker
  Debug Mode: false
  Registry: https://index.docker.io/v1/
  Labels:
  Experimental: true
  Insecure Registries:
   127.0.0.0/8
  Live Restore Enabled: true

 WARNING: No swap limit support

+ echo check-config.sh version: 2b0755b936416834e14208c6c37b36977e67ea35
 check-config.sh version: 2b0755b936416834e14208c6c37b36977e67ea35
 + curl -fsSL -o /home/ubuntu/workspace/moby_PR-42445/check-config.sh https://raw.githubusercontent.com/moby/moby/2b0755b936416834e14208c6c37b36977e67ea35/contrib/check-config.sh
 + bash /home/ubuntu/workspace/moby_PR-42445/check-config.sh
 �[31mwarning: /proc/config.gz does not exist, searching other paths for kernel config ...
 info: reading kernel config from /boot/config-5.4.0-1048-aws ...

 Generally Necessary:
 - cgroup hierarchy: properly mounted [/sys/fs/cgroup]
 - apparmor: enabled and tools installed
 - CONFIG_NAMESPACES: enabled
 - CONFIG_NET_NS: enabled
 - CONFIG_PID_NS: enabled
 - CONFIG_IPC_NS: enabled
 - CONFIG_UTS_NS: enabled
 - CONFIG_CGROUPS: enabled
 - CONFIG_CGROUP_CPUACCT: enabled
 - CONFIG_CGROUP_DEVICE: enabled
 - CONFIG_CGROUP_FREEZER: enabled
 - CONFIG_CGROUP_SCHED: enabled
 - CONFIG_CPUSETS: enabled
 - CONFIG_MEMCG: enabled
 - CONFIG_KEYS: enabled
 - CONFIG_VETH: enabled (as module)
 - CONFIG_BRIDGE: enabled (as module)
 - CONFIG_BRIDGE_NETFILTER: enabled (as module)
 - CONFIG_IP_NF_FILTER: enabled (as module)
 - CONFIG_IP_NF_TARGET_MASQUERADE: enabled (as module)
 - CONFIG_NETFILTER_XT_MATCH_ADDRTYPE: enabled (as module)
 - CONFIG_NETFILTER_XT_MATCH_CONNTRACK: enabled (as module)
 - CONFIG_NETFILTER_XT_MATCH_IPVS: enabled (as module)
 - CONFIG_NETFILTER_XT_MARK: enabled (as module)
 - CONFIG_IP_NF_NAT: enabled (as module)
 - CONFIG_NF_NAT: enabled (as module)
 - CONFIG_POSIX_MQUEUE: enabled

 Optional Features:
 - CONFIG_USER_NS: enabled
 - CONFIG_SECCOMP: enabled
 - CONFIG_SECCOMP_FILTER: enabled
 - CONFIG_CGROUP_PIDS: enabled
 - CONFIG_MEMCG_SWAP: enabled
 - CONFIG_MEMCG_SWAP_ENABLED: missing
     �[1;30m(cgroup swap accounting is currently not enabled, you can enable it by setting boot option "swapaccount=1")
 - CONFIG_BLK_CGROUP: enabled
 - CONFIG_BLK_DEV_THROTTLING: enabled
 - CONFIG_CGROUP_PERF: enabled
 - CONFIG_CGROUP_HUGETLB: enabled
 - CONFIG_NET_CLS_CGROUP: enabled (as module)
 - CONFIG_CGROUP_NET_PRIO: enabled
 - CONFIG_CFS_BANDWIDTH: enabled
 - CONFIG_FAIR_GROUP_SCHED: enabled
 - CONFIG_RT_GROUP_SCHED: missing
 - CONFIG_IP_NF_TARGET_REDIRECT: enabled (as module)
 - CONFIG_IP_VS: enabled (as module)
 - CONFIG_IP_VS_NFCT: enabled
 - CONFIG_IP_VS_PROTO_TCP: enabled
 - CONFIG_IP_VS_PROTO_UDP: enabled
 - CONFIG_IP_VS_RR: enabled (as module)
 - CONFIG_SECURITY_SELINUX: enabled
 - CONFIG_SECURITY_APPARMOR: enabled
 - CONFIG_EXT4_FS: enabled
 - CONFIG_EXT4_FS_POSIX_ACL: enabled
 - CONFIG_EXT4_FS_SECURITY: enabled
 - Network Drivers:
   - "overlay":
     - CONFIG_VXLAN: enabled (as module)
     - CONFIG_BRIDGE_VLAN_FILTERING: enabled
       Optional (for encrypted networks):
       - CONFIG_CRYPTO: enabled
       - CONFIG_CRYPTO_AEAD: enabled
       - CONFIG_CRYPTO_GCM: enabled
       - CONFIG_CRYPTO_SEQIV: enabled
       - CONFIG_CRYPTO_GHASH: enabled
       - CONFIG_XFRM: enabled
       - CONFIG_XFRM_USER: enabled (as module)
       - CONFIG_XFRM_ALGO: enabled (as module)
       - CONFIG_INET_ESP: enabled (as module)
   - "ipvlan":
     - CONFIG_IPVLAN: enabled (as module)
   - "macvlan":
     - CONFIG_MACVLAN: enabled (as module)
     - CONFIG_DUMMY: enabled (as module)
   - "ftp,tftp client in container":
     - CONFIG_NF_NAT_FTP: enabled (as module)
     - CONFIG_NF_CONNTRACK_FTP: enabled (as module)
     - CONFIG_NF_NAT_TFTP: enabled (as module)
     - CONFIG_NF_CONNTRACK_TFTP: enabled (as module)
 - Storage Drivers:
   - "aufs":
     - CONFIG_AUFS_FS: enabled (as module)
   - "btrfs":
     - CONFIG_BTRFS_FS: enabled (as module)
     - CONFIG_BTRFS_FS_POSIX_ACL: enabled
   - "devicemapper":
     - CONFIG_BLK_DEV_DM: enabled
     - CONFIG_DM_THIN_PROVISIONING: enabled (as module)
   - "overlay":
     - CONFIG_OVERLAY_FS: enabled (as module)
   - "zfs":
     - /dev/zfs: present
     - zfs command: missing
     - zpool command: missing

 Limits:
 - /proc/sys/kernel/keys/root_maxkeys: 1000000

 + true