various dockerfile improvements

• switch to java 15
• respect gradle version from repository
• switch to https://adoptopenjdk.net/

Rationale:

• java 14 is no longer supported and is not an LTS version
• using the gradle wrapper instead of the gradle image ensures that the gradle version in the image is the same as the gradle version when building locally. See https://docs.gradle.org/current/userguide/gradle_wrapper.html
• adoptopenjdk is just a personal preference over oracles openjdk builds