Add getContainerExtension method (#945) #1308

Job	Run time
setup-matrix	4s
static-analysis (8.1, lowest, false, false) / Static analysis with phpstan	1m 28s
static-analysis (8.4, highest, 12.x-dev as 12.99.9, true, true) / Static analysis with phpstan	50s
	2m 22s

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