cmd/atlas: refactor parse dockerURL logic #1004

Job	Run time
integration-maria102	1m 20s
integration-maria103	1m 28s
integration-maria107	1m 28s
integration-mysql56	1m 30s
integration-mysql57	1m 22s
integration-mysql8	1m 43s
integration-postgres-ext-postgis	1m 52s
integration-postgres10	1m 48s
integration-cockroach	1m 31s
integration-postgres11	1m 50s
integration-postgres12	1m 43s
integration-postgres13	3m 27s
integration-postgres14	1m 51s
integration-postgres15	1m 50s
integration-sqlite	1m 4s
integration-tidb5	2m 2s
integration-tidb6	1m 55s
	29m 44s

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