Fix GLIBC_2.38 mismatch: switch base image to ubuntu:24.04

CI runner (Ubuntu 24.04, glibc 2.39) produces binaries that require
GLIBC_2.38+. debian:bookworm-slim ships glibc 2.36 which doesn't have
the GLIBC_2.38 versioned symbols — container crashes immediately with
"version GLIBC_2.38 not found". Switch to ubuntu:24.04 (glibc 2.39)
to match the build environment. Also updates libcurl4/libssl3 package
names to their Ubuntu 24.04 canonical t64 forms.

Dockerfile.stage

@@ -14,12 +14,14 @@
 #   - soul-demo:  compiled by cc on host → dist/soul-demo
 #   - k3s:        downloaded by curl on host → dist/k3s
 
-FROM debian:bookworm-slim
+FROM ubuntu:24.04
+
+ENV DEBIAN_FRONTEND=noninteractive
 
 RUN apt-get update \
  && apt-get install -y --no-install-recommends \
-        libcurl4 \
-        libssl3 \
+        libcurl4t64 \
+        libssl3t64 \
         ca-certificates \
  && rm -rf /var/lib/apt/lists/* \
  && groupadd -r landing && useradd -r -g landing landing \