Bump docker/build-push-action from 3 to 6 #26

Job	Run time
linux_coverage (ubuntu-20.04, gcc)	50s
linux_coverage (ubuntu-22.04, gcc)	1m 18s
build_osx_autotools_default_args	1m 35s
linux (ubuntu-20.04, gcc)	1m 53s
linux_coverage (ubuntu-latest, gcc)	48s
linux_scanbuild (ubuntu-20.04)	2m 4s
linux (ubuntu-20.04, clang)	1m 12s
nix-matrix	13s
linux (ubuntu-22.04, gcc)	1m 11s
linux (ubuntu-22.04, clang)	1m 19s
linux (ubuntu-latest, gcc)	1m 14s
linux (ubuntu-latest, clang)	1m 21s
nix ("default")	54s
nix ("handlebars-c")	56s
nix ("handlebars-c-cmake")	39s
nix ("handlebars-c-minimal")	46s
nix ("handlebars-c-static")	53s
nix ("pre-commit-check")	26s
	19m 32s

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