[Github] Update libc container

Add git/cmake which are needed by the workflow. Add some comments pulled
from the libc workflow. Also use a user rather than running as root to
prevent file permissions tests from failing.

Author: boomanaiden154

.github/workflows/containers/libc/Dockerfile

@@ -1,7 +1,9 @@
 FROM docker.io/library/ubuntu:24.04
 
+# MPFR is required by some of the mathlib tests.
 RUN apt-get update && \
     apt-get install -y \
+    git \
     libmpfr-dev \
     libgmp-dev \
     libmpc-dev \
@@ -12,6 +14,7 @@ RUN apt-get update && \
     lsb-release \
     software-properties-common \
     gnupg \
+    cmake \
     linux-libc-dev && \
     apt-get clean && \
     rm -rf /var/lib/apt/lists/*
@@ -20,5 +23,20 @@ RUN wget https://apt.llvm.org/llvm.sh && \
     chmod +x llvm.sh && \
     sudo ./llvm.sh 21 && \
     rm llvm.sh
-
+    
+# Debian has a multilib setup, so we need to symlink the asm directory.
+# For more information, see https://wiki.debian.org/Multiarch/LibraryPathOverview
 RUN ln -sf /usr/include/$(uname -p)-linux-gnu/asm /usr/include/asm
+
+# Create a new user to avoid test failures related to a lack of expected
+# permissions issues in some tests. Set the user id to 1001 as that is the
+# user id that Github Actions uses to perform the checkout action.
+RUN useradd gha -u 1001 -m -s /bin/bash
+
+# Also add the user to passwordless sudoers so that we can install software
+# later on without having to rebuild the container.
+RUN adduser gha sudo
+RUN echo '%sudo ALL=(ALL) NOPASSWD:ALL' >> /etc/sudoers
+
+USER gha
+WORKDIR /home/gha