fix: preserve steamcmd sdk libraries

[MarcStdt]

Summary

• preserve upstream SteamCMD linux32/linux64 steamclient.so before removing the steam user
• seed the SDK libraries into the druid user Steam paths expected by LGSM/Steamworks servers
• set the steamcmd image workdir to /home/druid

Validation

• docker build --platform linux/amd64 -f Dockerfile.steamcmd --build-arg VERSION=v0.1.246 -t druid-steamcmd-sdk-test .
• docker run --rm --platform linux/amd64 --entrypoint sh druid-steamcmd-sdk-test -lc "test -s /home/druid/.steam/sdk64/steamclient.so; test -s /home/druid/.steam/sdk32/steamclient.so"

[druid-infra]

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