Bump LLVM version to 18

This commit changes clang installed directly from apt to clang installed
by an install LLVM apt script.
It installs LLVM18 (not reproducible).

To make it work easily, it was necessary to change Debian to Ubuntu.

Dockerfile

@@ -1,20 +1,26 @@
-FROM debian:12-slim
+FROM ubuntu:jammy
 
 RUN apt update && apt install -y \
     build-essential \
-    clang \
+    gnupg \
+    lsb-release \
     ruby \
     ruby-dev \
+    software-properties-common \
+    wget \
     && rm -rf /var/lib/apt/lists/*
 
+RUN wget https://apt.llvm.org/llvm.sh && chmod +x llvm.sh
+RUN ./llvm.sh 18
+
 ENV APP_DIR="/app"
 RUN mkdir $APP_DIR
 WORKDIR $APP_DIR
 
-ENV CC="clang"
-ENV CXX="clang++"
-ENV LDSHARED="clang -shared"
-ENV LDSHAREDXX="clang++ -shared"
+ENV CC="clang-18"
+ENV CXX="clang++-18"
+ENV LDSHARED="clang-18 -shared"
+ENV LDSHAREDXX="clang++-18 -shared"
 
 # The MAKE variable allows overwriting the make command at runtime. This forces the
 # Ruby C extension to respect ENV variables when compiling, like CC, CFLAGS, etc.