mongo:8 was crashing with exit code 139 (SIGSEGV) on the Hetzner CAX11
ARM64 instance. Switch to mongo:7 (LTS) which has more stable ARM64
support. Also remove the --wiredTigerCacheSizeGB=0.25 arg since the
512Mi memory limit already bounds memory use adequately.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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