diff --git a/.github/workflows/build.yml b/.github/workflows/build.yml
index f235aaa..3162ab1 100644
--- a/.github/workflows/build.yml
+++ b/.github/workflows/build.yml
@@ -14,11 +14,10 @@ jobs:
       matrix:
         # https://www.ruby-lang.org/en/downloads/branches/
         ruby-version:
-          - "3.0"
-          - "3.1"
           - "3.2"
           - "3.3"
           - "3.4"
+          - "4.0"
     steps:
       - uses: actions/checkout@v4
       - name: Set up Ruby ${{ matrix.ruby-version }}
diff --git a/Dockerfile b/Dockerfile
index c4655d4..1333c2f 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -1,5 +1,5 @@
 # https://hub.docker.com/_/ruby
-ARG RUBY_VERSION=3.4
+ARG RUBY_VERSION=4.0
 
 FROM ruby:$RUBY_VERSION-slim-bookworm
 
@@ -8,9 +8,9 @@ RUN apt update && apt install -y \
     wget \
     && rm -rf /var/lib/apt/lists/*
 
-# LLVM builds version 15-19 for Debian 12 (Bookworm)
+# LLVM builds version 15-21 for Debian 12 (Bookworm)
 # https://apt.llvm.org/bookworm/dists/
-ARG LLVM_VERSION=19
+ARG LLVM_VERSION=21
 
 RUN echo "deb http://apt.llvm.org/bookworm/ llvm-toolchain-bookworm-$LLVM_VERSION main" > /etc/apt/sources.list.d/llvm.list
 RUN echo "deb-src http://apt.llvm.org/bookworm/ llvm-toolchain-bookworm-$LLVM_VERSION main" >> /etc/apt/sources.list.d/llvm.list