Use eclipse-temurin:21 as base image

[joggee-fr]

Replace deprecated openjdk:17-jdk-slim-bullseye base image as now considered as deprecated. The eclipse-temurin:21 is based on Ubuntu 24.04 and the Debian package could still be used just like before.

1. Just add adduser to installed packages as needed by the Openfire postinst script.
2. Retrieve Java binary using $JAVA_HOME environment variable.

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Done some quick tests, setup the server with embedded database, access the admin console and create a test user through XMPP.

Issue describe in PR #6 is still present but it is a first step to upgrade the current Openfire image.
Next step could be to use (very) recent Openfire 4.9.0 release.

[dereckson]

Just a note I've tested 4.9.0 and we've an issue with the Base64 class replacement.

Issue: https://devcentral.nasqueron.org/T2022

Upgrade: https://devcentral.nasqueron.org/D3457

I can still push it to a non-latest 4.9.0 tag if you want to test it.

[joggee-fr]

I have just added a commit to update README.md.
The goal of this PR is just to use an up-to-date OpenJDK base image. Eclipse Temurin project looks like an interesting solution to do this and Java 21 is the last LTS for now.

I hope you could give some tests to my proposition and next, we could investigate on issues regarding Openfire 4.9.0.
BTW, after a very very quick try to use 4.9.0 with eclipse-temurin:21, I didn't (yet?) stumble open the issue you have reported.

[dereckson]

Any specific reason to choose specifically Eclipse Temurin as OpenJDK vendor?

[joggee-fr]

There is no technical reason for Eclipse Temurin among the other alternatives proposed for OpenJDK Docker images. But if I'm right this is the only one supported by a foundation. BTW, the image is based on Ubuntu, a Debian derivative, and allows to keep the same logic for Openfire image build, at least for now.