Make CI checks work again by using the correct debian versions that have the expected GLIBC versions.

[ArSn]

Problem 1 - buster-slim test failures (GLIBC_2.29 not found):

• The binary is built on Debian Bullseye (GLIBC 2.31), but Buster only ships GLIBC 2.28
• Debian Buster reached EOL in June 2024
• Fix: Replaced debian:buster-slim with debian:bookworm-slim (Debian 12) in the test matrix

Problem 2 - build-debian (24) Docker build failure:

• A transient network error (Connection reset by peer) caused apt-get install to fail
• Fix: Added --fix-missing flag and a retry fallback — if the first install attempt fails, it re-runs apt-get update and retries the install
• Also fixed the two legacy ENV key value format warnings to use ENV key=value