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

changed the description

mentioned in merge request !8 (merged)

created merge request !8 (merged) to address this issue

added Focus label

marked the checklist item switch to java 15 as completed

marked the checklist item respect gradle version from repository as completed

marked the checklist item switch to https://adoptopenjdk.net/ as completed

changed the description

closed with merge request !8 (merged)

mentioned in commit fddb9a49

reopened

closed

removed Focus label