Make image tag configurable via env var

...-benchmarks/docker-test/uc1-beam-flink/docker-compose.yml

...-benchmarks/docker-test/uc1-beam-samza/docker-compose.yml

...s/docker-test/uc1-flink-docker-compose/docker-compose.yml

...ocker-test/uc1-kstreams-docker-compose/docker-compose.yml

...-benchmarks/docker-test/uc2-beam-flink/docker-compose.yml

...-benchmarks/docker-test/uc2-beam-samza/docker-compose.yml

...s/docker-test/uc2-flink-docker-compose/docker-compose.yml

...ocker-test/uc2-kstreams-docker-compose/docker-compose.yml

...-benchmarks/docker-test/uc3-beam-flink/docker-compose.yml

...-benchmarks/docker-test/uc3-beam-samza/docker-compose.yml

...s/docker-test/uc3-flink-docker-compose/docker-compose.yml

...ocker-test/uc3-kstreams-docker-compose/docker-compose.yml

...-benchmarks/docker-test/uc4-beam-flink/docker-compose.yml

...-benchmarks/docker-test/uc4-beam-samza/docker-compose.yml

...s/docker-test/uc4-flink-docker-compose/docker-compose.yml

...ocker-test/uc4-kstreams-docker-compose/docker-compose.yml