Prevent GDB debuginfod startup hangs

[JoelYYoung]

Summary

• remove Ubuntu debuginfod URL files from the image so cpptools 1.32.x cannot block on remote symbol downloads
• keep locally installed libc6-dbg symbols available
• remove the launch.json workaround that cpptools 1.32.x overrides
• remove the ineffective runtime-permission metadata label; the required docker run flags are now documented in the Wiki

Verification

• docker build --check .
• reproduced the hang with the published ARM64 image and debuginfod enabled (timed out after 20 seconds)
• removed /etc/debuginfod/*.urls in an otherwise identical temporary container and GDB reached main immediately

Related upstream regression: microsoft/vscode-cpptools#14458