Add veth interface for the container:
/interface/veth/add name=veth1 address=172.17.0.2/24 gateway=172.17.0.1
Create a bridge for containers and add veth to it:
/interface/bridge/add name=dockers
/ip/address/add address=172.17.0.1/24 interface=dockers
/interface/bridge/port add bridge=dockers interface=veth1
Setup NAT for outgoing traffic:
/ip/firewall/nat/add chain=srcnat action=masquerade src-address=172.17.0.0/24