fix: add steamcmd pz dependencies

[MarcStdt]

Summary

• use artifacts.druid.gg as the SteamCMD Dockerfile base image
• add LGSM-required Project Zomboid runtime dependencies to the SteamCMD image

Test plan

• git diff --check
• CI Docker build validates package availability and image build

Notes

• no runtime behavior change; this only hardens the shared SteamCMD worker image after PZ prebuild exposed missing dependencies

[druid-infra]

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