Refactor docker workflow for better compatibility #46

Job	Run time
Docker	1m 7s
Linters (8.1)	40s
Linters (8.2)	40s
Phar (8.1)	31s
Linters (8.3)	49s
Phar (8.2)	31s
PHPUnit (8.1, xdebug, --prefer-lowest)	30s
Reports (8.1)	59s
PHPUnit (8.1, xdebug)	50s
Phar (8.3)	32s
Reports (8.2)	33s
PHPUnit (8.1, none, --prefer-lowest)	25s
Reports (8.3)	30s
PHPUnit (8.1, none)	22s
PHPUnit (8.2, xdebug, --prefer-lowest)	33s
PHPUnit (8.2, xdebug)	49s
PHPUnit (8.2, none, --prefer-lowest)	38s
PHPUnit (8.2, none)	22s
PHPUnit (8.3, xdebug, --prefer-lowest)	35s
PHPUnit (8.3, xdebug)	48s
PHPUnit (8.3, none, --prefer-lowest)	29s
PHPUnit (8.3, none)	25s
	13m 38s

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