Update Dockerfile and streamline build steps #5657

Job	Run time
build-all	3m 20s
tests (es, -Pjava-11, -Pelasticsearch60, es60, 11)	29m 20s
tests (es, -Pelasticsearch60, es60-java8, 8)	29m 30s
tests (es, -Pjava-11, -Pelasticsearch9, es9, 11)	31m 29s
tests (es, -Pelasticsearch8, es8-java8, 8)	31m 38s
tests (es, -Pjava-11, -Pelasticsearch8, es8, 11)	31m 33s
tests (es, -Pelasticsearch7, es7-java8, 8)	30m 24s
tests (es, -Pjava-11, -Pelasticsearch7, es7, 11)	30m 38s
tests (es, -Pjava-11, -Pelasticsearch6, es6, 11)	30m 10s
tests (es, -Pelasticsearch6, es6-java8, 8)	29m 46s
tests (es, -Pelasticsearch9, es9-java8, 8)	31m 24s
	5h 9m 12s

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