ci(fedora41): split install prerequisites and build RBEnv into separate phases

It improves parallelization and speeds up the build by reusing cached layers.

.gitlab/ci/docker/fedora41/Dockerfile

@@ -7,6 +7,7 @@ FROM ${BASE_IMAGE} AS dnf-cache
 RUN --mount=type=bind,source=dnf.conf,target=/etc/dnf/dnf.conf \
     --mount=type=bind,source=deps_packages.lst,target=/root/deps_packages.lst \
     --mount=type=bind,source=iwyu_packages.lst,target=/root/iwyu_packages.lst \
+    --mount=type=bind,source=rbenv_packages.lst,target=/root/rbenv_packages.lst \
     --mount=type=bind,source=rust_packages.lst,target=/root/rust_packages.lst \
     --mount=type=bind,source=rvm_packages.lst,target=/root/rvm_packages.lst \
     --mount=type=tmpfs,target=/var/log \
@@ -48,6 +49,27 @@ RUN --mount=type=bind,source=build_rvm.sh,target=/root/build_rvm.sh \
     sh /root/build_rvm.sh
 
 
+FROM ${BASE_IMAGE} AS rbenv
+# Pre-install prerequisites for `rbenv`.
+RUN --mount=type=bind,source=dnf.conf,target=/etc/dnf/dnf.conf \
+    --mount=type=bind,source=rbenv_packages.lst,target=/root/rbenv_packages.lst \
+    --mount=type=cache,from=dnf-cache,source=/var/cache/libdnf5,target=/var/cache/libdnf5,sharing=private \
+    --mount=type=tmpfs,target=/var/log \
+    --mount=type=tmpfs,target=/tmp \
+    dnf install -y $(grep '^[^#]\+$' /root/rbenv_packages.lst)
+
+
+FROM rbenv AS rbenv-build
+ENV RBENV_ROOT=/opt/rbenv
+ENV RBENV_BUILD_ROOT=/root/.cache/rbenv-build
+ENV RUBY_BUILD_CACHE_PATH=/root/.cache/rbenv
+# Build Ruby with `rbenv`.
+RUN --mount=type=cache,target=/root/.cache \
+    --mount=type=bind,source=build_rbenv.sh,target=/root/build_rbenv.sh \
+    --mount=type=tmpfs,target=/tmp \
+    sh /root/build_rbenv.sh
+
+
 FROM ${BASE_IMAGE} AS iwyu-build-env
 LABEL maintainer="Kyle Edwards <kyle.edwards@kitware.com>"
 # Pre-install prerequisites to build IWYU.
@@ -98,3 +120,6 @@ RUN --mount=type=bind,from=rust-build,source=/root,target=/root \
 
 RUN --mount=type=bind,from=rvm-build,source=/root,target=/root \
     tar -C /usr/local -xf /root/rvm.tar
+
+RUN --mount=type=bind,from=rbenv-build,source=/root,target=/root \
+    tar -C / -xf /root/rbenv.tar && rbenv global 3.1.2

.gitlab/ci/docker/fedora41/build_rbenv.sh

@@ -0,0 +1,28 @@
+#!/bin/sh
+
+set -e
+
+echo "gem: --no-document" > ~/.gemrc
+
+# Ruby rbenv
+export RUBY_CONFIGURE_OPTS=--disable-install-doc
+export RUBY_BUILD_CURL_OPTS=-C-
+rbenv install 3.1.2 -k -s -v
+
+cat <<EOF >/tmp/exclude.lst
+*LICENSE*
+*/doc/*
+*/man/*
+*.md
+BSDL
+CONTRIBUTING.*
+COPYING
+LEGAL
+PSFL
+README.rdoc
+History.rdoc
+gem_make.out
+test-unit-*/test
+rss-*/test
+EOF
+tar -cf /root/rbenv.tar --exclude-from=/tmp/exclude.lst ${RBENV_ROOT}

.gitlab/ci/docker/fedora41/install_deps.sh

@@ -11,7 +11,3 @@ done
 
 # Remove some other packages tests
 find /usr/lib64/python3.13/site-packages/breezy -type d -a -name tests -exec rm -rf {} +
-
-# Ruby rbenv
-rbenv install 3.1.2
-rbenv global 3.1.2

.gitlab/ci/docker/fedora41/rbenv_packages.lst

@@ -0,0 +1,18 @@
+rbenv
+ruby-build-rbenv
+
+# Packages needed for `rbenv`
+# https://github.com/rbenv/ruby-build/wiki#fedora
+autoconf
+bzip2
+gcc
+gdbm-devel
+libffi-devel
+libyaml-devel
+make
+ncurses-devel
+openssl-devel
+patch
+readline-devel
+rust
+zlib-devel