Use Proton root for Docker user override

[BharatDeva]

PR checklist:

• Did you run ClangFormat ? Not applicable; this changes a shell script.
• Did you separate headers to a different section in existing community code base ? Not applicable.
• Did you surround proton: starts/ends for new code in existing community code base ? Not applicable for the Docker entrypoint shell script.

Please write user-readable short description of the changes:

This updates the Docker entrypoint path that handles PROTON_USER, PROTON_PASSWORD, and PROTON_DEFAULT_ACCESS_MANAGEMENT.

When those environment variables are provided, the entrypoint now writes a users.d/default-user.xml override using Proton's <proton> root element. That matches the root used by the Proton server config and avoids the config merge failure reported when Docker env-based user configuration is enabled.

The generated XML also escapes env-derived values and validates the username before using it as an XML element name.

Validation performed:

wsl -e bash -n docker/server/entrypoint.sh

Result: shell syntax check passed.

git show --check --oneline --summary HEAD

Result:

32d49afa Use Proton root for Docker user override

Fixes #927

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