contributing: add paragraph regarding the container image #3999

Job	Run time
windows-mingw-build (32)	1m 8s
macos-static	2m 4s
windows-mingw-build (64)	1m 0s
windows (32, windows-2019)	6m 11s
unix (ubuntu-20.04, g++, make)	5m 45s
windows (32, windows-2022)	6m 7s
windows (64, windows-2019)	5m 24s
unix (ubuntu-20.04, g++, cmake)	5m 40s
windows (64, windows-2022)	4m 32s
unix (ubuntu-20.04, clang++, make)	5m 59s
unix (ubuntu-20.04, clang++, cmake)	6m 3s
unix (ubuntu-22.04, g++, make)	6m 0s
unix (ubuntu-22.04, g++, cmake)	5m 59s
unix (ubuntu-22.04, clang++, make)	3m 59s
unix (ubuntu-22.04, clang++, cmake)	4m 7s
unix (macos-14, clang++, make)	6m 45s
unix (macos-14, clang++, cmake)	5m 54s
windows-mingw-testing (windows-2019, 32)	4m 9s
windows-mingw-testing (windows-2019, 64)	4m 10s
windows-mingw-testing (windows-2022, 32)	4m 56s
windows-mingw-testing (windows-2022, 64)	3m 54s
	1h 39m 46s

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