Update docker base image to openjdk:8u275-jre-slim-buster (fixes #363)
Update docker base image to openjdk:8u275-jre-slim-buster (fixes #363)

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