Build gcc11 builds in container. #422

Job	Run time
B2 msvc-14.2	8m 58s
B2 msvc-14.3	7m 18s
CMake msvc-14.2 Debug C++17	9m 26s
CMake gcc-11 17 Debug	32m 55s
CMake msvc-14.2 Release C++17	5m 26s
CMake gcc-11 20 Release	31m 54s
CMake msvc-14.3 Debug C++20	5m 29s
CMake clang-11 17 Debug	4m 49s
CMake msvc-14.3 Release C++20	5m 40s
CMake clang-11 20 Debug	5m 37s
CMake clang-13 17 Release	7m 45s
CMake clang-13 20 Release	6m 0s
CMake clang-14, 17 Debug -stdlib=libc++	1m 18s
CMake clang-14 20 Release -stdlib=libc++	9m 10s
	2h 21m 45s

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