Currently, the Alpine Linux Docker images are about the same size of the Ubuntu images, sometimes slightly bigger. As the value of Alpine is in great part its lower footprint, having bigger Alpine images somewhat defeats their purpose.

The Dockerfile used by Atlassian for Alpine and Ubuntu respectively use:
Alpine: FROM adoptopenjdk/openjdk8:alpine
Ubuntu: FROM adoptopenjdk/openjdk8:slim

The 'slim' version of adoptopenjdk is important. There is a 'slim' version for Alpine but Atlassian does not seem to use it:
adoptopenjdk/openjdk8:alpine-slim

I think this is valuable improvement to the Alpine images for Confluence (saves 150M).