Cache docker when using Github actions #15449

Job	Run time
cancel	4s
Check if image exists in registry	9s
licensing	5m 8s
pyflakes	4m 18s
linux (ubuntu-20.04, debug)	1h 25m 58s
linux-fpga (ubuntu-20.04, debug)	1h 47m 5s
linux (ubuntu-20.04, release)	1h 2m 41s
linux (ubuntu-20.04, codecov)	1h 29m 7s
Build image	0s
Build SLES image	0s
cppcheck	11m 0s
tidy	2m 13s
sles	22m 44s
format	2m 22s
	6h 32m 49s

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