Update docker base image to openjdk:8u272-jre-slim-buster (fixes #353)
Update docker base image to openjdk:8u272-jre-slim-buster (fixes #353)

greenmail-mail-test/greenmail,marcelmay/greenmail,marcelmay/greenmail,marcelmay/greenmail,greenmail-mail-test/greenmail,greenmail-mail-test/greenmail,marcelmay/greenmail,marcelmay/greenmail