Bump docker/build-push-action from 5 to 6 #1095

Job	Run time
quicktests	33s
lint	19s
unittests (ubuntu-22.04)	1m 39s
unittests (self-hosted, linux, arm64)	6m 46s
coverage	1m 1s
unittests (macos-13)	2m 11s
unittests (self-hosted, macos, arm64)	49s
unittests (windows-2022)	3m 5s
Build postcli (ubuntu-22.04)	36s
Build postcli (self-hosted, linux, arm64)	20s
Build postcli (macos-13)	1m 22s
Build postcli (self-hosted, macos, arm64)	26s
Build postcli (windows-2022)	3m 21s
Publish release	0s
	22m 28s

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