tt CLI (2.11): upgrade internal docker container

[TarantoolBot]

Product: tt CLI
Since: tt 2.11.1
Root document: https://www.tarantool.io/en/doc/latest/tooling/tt_cli/install/ https://www.tarantool.io/en/doc/latest/tooling/tt_cli/pack/

SME: @ oleg-jukovec @ elhimov

Details

Docker container that is used when in-docker tarantool build is
requested was upgraded from Ubuntu 18.04 to Ubuntu 20.04.
Requested by @elhimov in tarantool/tt@222e2fc.