CI: Restore building MakeCode with Docker. #257

Job	Run time
ubuntu-20.04, gcc 7-2017-q4, cmake 3.6.0	50s
ubuntu-20.04, gcc 7-2017-q4, cmake latest	42s
ubuntu-20.04, gcc latest, cmake 3.6.0	1m 2s
ubuntu-20.04, gcc latest, cmake latest	1m 0s
macos-11, gcc 7-2017-q4, cmake 3.6.0	1m 51s
macos-11, gcc 7-2017-q4, cmake latest	1m 45s
macos-11, gcc latest, cmake 3.6.0	1m 58s
macos-11, gcc latest, cmake latest	2m 0s
windows-2019, gcc 7-2017-q4, cmake 3.6.0	2m 38s
windows-2019, gcc 7-2017-q4, cmake latest	2m 12s
windows-2019, gcc latest, cmake 3.6.0	2m 28s
windows-2019, gcc latest, cmake latest	3m 5s
	21m 31s

Provide feedback