Dockerfile: use multi-stage build

[frafl]

Problem:

The httpd:2.4 Docker container is now based on boookworm, which does no longer contain openjdk-11-jre-headless. Sencha's ant based build tool requires Nashorn to process .xml files containing JavaScript, but Nashorn was removed in JRE15, so building with the current Dockerfile throws an error.

Solution(s):

This PR contains 3 commits which all solve the problem in different ways:

• The first one simply adds an external source for JRE 11.
• The second one splits the process into two stages both based on httpd:2.4. Only the first one requires the external source for JRE 11 and the second one simply copies the files under /usr/local/lada (except for the vhosts file).
• The third commit uses the Debian image debian:11-slim (i.e., bullseye) instead of httpd:2.4. This ensures that the package openjdk-11-jre-headless is still available without external resources.

The two intermediate commits are intentionally part of this PR to pinpoint potential issues.