docker: add to omnia

Alpine linux build script: https://git.alpinelinux.org/cgit/aports/tree/community/docker/APKBUILD?h=3.7-stable

Forum: https://forum.turris.cz/t/docker-on-turris-omnia/242/37

Output from https://github.com/moby/moby/blob/master/contrib/check-config.sh

root@turris:/# ./check-config.sh 
info: reading kernel config from /proc/config.gz ...

Generally Necessary:
- cgroup hierarchy: single mountpoint! [/sys/fs/cgroup]
    (see https://github.com/tianon/cgroupfs-mount)
- 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
- CONFIG_BRIDGE_NETFILTER: enabled
- CONFIG_NF_NAT_IPV4: 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: missing
- CONFIG_IP_NF_NAT: enabled (as module)
- CONFIG_NF_NAT: enabled (as module)
- CONFIG_NF_NAT_NEEDED: enabled
- CONFIG_POSIX_MQUEUE: enabled
- CONFIG_DEVPTS_MULTIPLE_INSTANCES: enabled

Optional Features:
- CONFIG_USER_NS: enabled
- CONFIG_SECCOMP: missing
- CONFIG_CGROUP_PIDS: missing
- CONFIG_MEMCG_SWAP: enabled
- CONFIG_MEMCG_SWAP_ENABLED: missing
    (cgroup swap accounting is currently not enabled, you can enable it by setting boot option "swapaccount=1")
- CONFIG_MEMCG_KMEM: missing
- CONFIG_BLK_CGROUP: enabled
- CONFIG_BLK_DEV_THROTTLING: missing
- CONFIG_IOSCHED_CFQ: missing
- CONFIG_CFQ_GROUP_IOSCHED: missing
- CONFIG_CGROUP_PERF: missing
- CONFIG_CGROUP_HUGETLB: missing
- CONFIG_NET_CLS_CGROUP: enabled
- CONFIG_CGROUP_NET_PRIO: missing
- CONFIG_CFS_BANDWIDTH: missing
- CONFIG_FAIR_GROUP_SCHED: missing
- CONFIG_RT_GROUP_SCHED: missing
- CONFIG_IP_VS: missing
- CONFIG_IP_VS_NFCT: missing
- CONFIG_IP_VS_RR: missing
- CONFIG_EXT3_FS: enabled
- CONFIG_EXT3_FS_XATTR: missing
- CONFIG_EXT3_FS_POSIX_ACL: missing
- CONFIG_EXT3_FS_SECURITY: missing
    (enable these ext3 configs if you are using ext3 as backing filesystem)
- CONFIG_EXT4_FS: enabled
- CONFIG_EXT4_FS_POSIX_ACL: enabled
- CONFIG_EXT4_FS_SECURITY: enabled
- Network Drivers:
  - "overlay":
    - CONFIG_VXLAN: enabled (as module)
      Optional (for encrypted networks):
      - CONFIG_CRYPTO: enabled
      - CONFIG_CRYPTO_AEAD: enabled (as module)
      - CONFIG_CRYPTO_GCM: enabled (as module)
      - CONFIG_CRYPTO_SEQIV: enabled (as module)
      - CONFIG_CRYPTO_GHASH: enabled (as module)
      - CONFIG_XFRM: enabled
      - CONFIG_XFRM_USER: enabled (as module)
      - CONFIG_XFRM_ALGO: enabled (as module)
      - CONFIG_INET_ESP: enabled (as module)
      - CONFIG_INET_XFRM_MODE_TRANSPORT: enabled (as module)
  - "ipvlan":
    - CONFIG_IPVLAN: missing
  - "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: missing
  - "btrfs":
    - CONFIG_BTRFS_FS: enabled
    - CONFIG_BTRFS_FS_POSIX_ACL: enabled
  - "devicemapper":
    - CONFIG_BLK_DEV_DM: enabled (as module)
    - CONFIG_DM_THIN_PROVISIONING: missing
  - "overlay":
    - CONFIG_OVERLAY_FS: enabled
  - "zfs":
    - /dev/zfs: missing
    - zfs command: missing
    - zpool command: missing

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