Add aws-user and github-user to CI docker images, drop --user root

[Fedr]

All CI docker images now end with the same two non-root users, and github-user (UID 1001) is the default:

RUN useradd -u 8877 -m aws-user
RUN useradd -u 1001 -m github-user
USER github-user

UID 1001 matches the runner user on GitHub-hosted runners, so job containers can write to the runner-mounted directories (_runner_file_commands, workspace) without running as root; aws-user keeps UID 8877 available for self-hosted runners via an explicit --user aws-user.

Accordingly, options: --user root is removed from all workflows (build-test-ubuntu-x64, build-test-ubuntu-arm64, build-test-linux-vcpkg, generate-c-bindings, update-docs-manual), and the container-options matrix plumbing in pip-build and release-tests is dropped entirely — every container now runs as its image's default user. For the stock distro images used in pip tests this is no behavior change (their default user is already root); MeshLib images now run as github-user.

Changes for the previously existing users:

• ubuntu22/ubuntu24 had user (UID 8877, default): renamed to aws-user, default is now github-user.
• emscripten/emscripten-build-c-bindings had user (UID 1001, default): renamed to github-user, same UID, so the effective runtime user is unchanged.
• emscripten-generate-c-bindings and rockylinux8/9-vcpkg had no user (ran as root, or with --user root): they now default to github-user.

[Grantim]

do we need this option at all?

[Grantim]

same