Test mutiple target PHP versions via docker and fixes some bugs

[sj-i]

Also fixes several bugs found on this way.
Because target VMs in docker containers are not children of the invoker process, we cannot simply attach them from the PHPUnit process.
So I have moved the PHPUnit process itself to a docker container too and given it CAP_SYS_PTRACE.

Maybe we should cache images for speeding up testing in CI.

[coveralls]

Pull Request Test Coverage Report for Build 7815295514

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A quick fix for this PR: rebase it; your next report should be accurate.

• 45 of 60 (75.0%) changed or added relevant lines in 6 files are covered.
• 6 unchanged lines in 3 files lost coverage.
• Overall coverage increased (+2.1%) to 86.214%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/Lib/PhpInternals/Types/Zend/V74/ZendArray.php	1	16	6.25%

Files with Coverage Reduction	New Missed Lines	%
src/Lib/Elf/SymbolResolver/Elf64LinearScanSymbolResolver.php	1	85.71%
src/Lib/PhpInternals/Types/Zend/ZendExecuteData.php	1	93.78%
src/Lib/PhpProcessReader/PhpMemoryReader/MemoryLocation/ZendMmChunkMemoryLocation.php	4	71.43%

Totals
Change from base Build 7676075003:	2.1%
Covered Lines:	5372
Relevant Lines:	6231

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💛 - Coveralls