bz2 extension could be interesting, as some tools are shipped as PHAR archives compressed in this format.
In the nasqueron/jenkins-agent-php image:
[pdepend] Fatal error: Uncaught exception 'PharException' with message 'bz2 extension is required for bzip2 compressed .phar file "/opt/pdepend.phar"' in /opt/pdepend.phar on line 45