[docker] move to ubuntu 20.04 (#595)
[docker] move to ubuntu 20.04 (#595)


openthread/borderrouter,openthread/ot-br-posix,jwhui/borderrouter,jwhui/borderrouter,bukepo/borderrouter,openthread/ot-br-posix,openthread/ot-br-posix,jwhui/borderrouter,openthread/ot-br-posix,bukepo/borderrouter,openthread/borderrouter,openthread/borderrouter,jwhui/borderrouter,bukepo/borderrouter,openthread/borderrouter,openthread/ot-br-posix,openthread/ot-br-posix,jwhui/borderrouter,openthread/borderrouter,bukepo/borderrouter,openthread/borderrouter,bukepo/borderrouter