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

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

.gitlab/ci/docker/fedora41/Dockerfile

@@ -37,16 +37,23 @@ RUN --mount=type=bind,source=dnf.conf,target=/etc/dnf/dnf.conf \
     sh /root/install_rvm.sh
 
 
-FROM ${BASE_IMAGE} AS iwyu-build
+FROM ${BASE_IMAGE} AS iwyu-build-env
 LABEL maintainer="Kyle Edwards <kyle.edwards@kitware.com>"
-
+# Pre-install prerequisites to build IWYU.
 RUN --mount=type=bind,source=dnf.conf,target=/etc/dnf/dnf.conf \
-    --mount=type=bind,source=install_iwyu.sh,target=/root/install_iwyu.sh \
     --mount=type=bind,source=iwyu_packages.lst,target=/root/iwyu_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 \
-    sh /root/install_iwyu.sh
+    dnf install -y $(grep '^[^#]\+$' /root/iwyu_packages.lst)
+
+
+FROM iwyu-build-env AS iwyu-build
+LABEL maintainer="Kyle Edwards <kyle.edwards@kitware.com>"
+# Build IWYU.
+RUN --mount=type=bind,source=build_iwyu.sh,target=/root/build_iwyu.sh \
+    --mount=type=tmpfs,target=/tmp \
+    sh /root/build_iwyu.sh
 
 
 FROM ${BASE_IMAGE} AS p4-dl

.gitlab/ci/docker/fedora41/build_iwyu.sh

@@ -2,9 +2,6 @@
 
 set -e
 
-# Install development tools.
-dnf install -y $(grep '^[^#]\+$' /root/iwyu_packages.lst)
-
 cd /root
 git clone "https://github.com/include-what-you-use/include-what-you-use.git"
 cd include-what-you-use