CI: Pre-bake curl, ca-certificates, and python3 into smoke build Dockerfiles

Include curl, ca-certificates, and python3 in the slim smoke test build container images (Ubuntu 24.04 amd64, Ubuntu 24.04 arm64, and Ubuntu 26.04 riscv64 cross-compile).

This ensures containers running with EIGEN_CI_SKIP_APT=true can download compiler caching tools and interact with authentication endpoints without invoking apt-get during CI jobs.
diff --git a/ci/docker/ubuntu-24.04-amd64-smoketest-build/Dockerfile b/ci/docker/ubuntu-24.04-amd64-smoketest-build/Dockerfile
index 9e796b3..a163c6e 100644
--- a/ci/docker/ubuntu-24.04-amd64-smoketest-build/Dockerfile
+++ b/ci/docker/ubuntu-24.04-amd64-smoketest-build/Dockerfile
@@ -14,7 +14,7 @@
 
 RUN apt-get update -y && \
     apt-get install -y --no-install-recommends \
-      cmake ninja-build git ccache \
+      cmake ninja-build git ccache curl ca-certificates python3 \
       gcc-10 g++-10 \
       g++-10-aarch64-linux-gnu \
       clang-14 \
diff --git a/ci/docker/ubuntu-24.04-arm64-smoketest-build/Dockerfile b/ci/docker/ubuntu-24.04-arm64-smoketest-build/Dockerfile
index f6c1d63..e091887 100644
--- a/ci/docker/ubuntu-24.04-arm64-smoketest-build/Dockerfile
+++ b/ci/docker/ubuntu-24.04-arm64-smoketest-build/Dockerfile
@@ -14,7 +14,7 @@
 
 RUN apt-get update -y && \
     apt-get install -y --no-install-recommends \
-      cmake ninja-build git ccache \
+      cmake ninja-build git ccache curl ca-certificates python3 \
       gcc-10 g++-10 \
       g++-10-x86-64-linux-gnu \
       clang-14 \
diff --git a/ci/docker/ubuntu-26.04-riscv64-smoketest-build/Dockerfile b/ci/docker/ubuntu-26.04-riscv64-smoketest-build/Dockerfile
index 15a5332..d27f28a 100644
--- a/ci/docker/ubuntu-26.04-riscv64-smoketest-build/Dockerfile
+++ b/ci/docker/ubuntu-26.04-riscv64-smoketest-build/Dockerfile
@@ -19,7 +19,7 @@
 
 RUN apt-get update -y && \
     apt-get install -y --no-install-recommends \
-      cmake ninja-build git ccache \
+      cmake ninja-build git ccache curl ca-certificates python3 \
       g++-15-riscv64-linux-gnu \
       clang-21 \
     && apt-get clean \