fix: docker/Dockerfile.gcc12 to reduce vulnerabilities

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/SNYK-UBUNTU2304-GLIBC-5919743
- https://snyk.io/vuln/SNYK-UBUNTU2304-GLIBC-5919743
- https://snyk.io/vuln/SNYK-UBUNTU2304-PAM-5463619
- https://snyk.io/vuln/SNYK-UBUNTU2304-PAM-5463619
- https://snyk.io/vuln/SNYK-UBUNTU2304-XZUTILS-5854648

docker/Dockerfile.gcc12

@@ -27,7 +27,7 @@ RUN cmake -D$target=ON  -DBUILD_CMD_LINE_TOOLS=ON -DBUILD_DEMONSTRATION=OFF .. &
 # RELEASE stage
 #FROM alpine:latest as release    # hitting a segfault during startup of some playground programs
 #FROM debian:buster-slim as release
-FROM ubuntu:lunar-20231004 as release
+FROM ubuntu:mantic-20231011 as release
 LABEL Theodore Omtzigt
 
 #RUN apk add --no-cache libc6-compat libstdc++ cmake make bash gawk sed grep bc coreutils