ci: prebuild the fedora toolchain as a container image

dockerfile

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+# Build environment for the RPM autoupdater.
+#
+# Rebuilt monthly by .gitea/workflows/build-image.yaml and pushed as :latest, so
+# the nightly package jobs start with the toolchain already in place instead
+# of each running a full dnf transaction first.
+#
+# Fedora specifically: rpmbuild, rpmspec and rpmlint are native there and
+# current. Packages are built with `dist` undefined, so the noarch result is not
+# tied to the Fedora release that built it and installs on EL and openSUSE too.
+FROM fedora:latest
+
+LABEL org.opencontainers.image.title="rpm-autoupdate"
+LABEL org.opencontainers.image.description="Fedora build environment for the RPM package autoupdater"
+LABEL org.opencontainers.image.source="https://git.bjphoster.com/rpm/.autoupdate"
+
+# nodejs       actions/checkout is a JavaScript action; the runner executes it
+#              with `node` from inside this image, and fedora ships none.
+# rpm-build    rpmbuild and rpmspec
+# rpmdevtools  spectool, to resolve Source0's file name
+# rpmlint      advisory lint of the spec and the built package
+# util-linux   runuser, to drop from root to the build user
+# make         the usual BuildRequires of a script package
+RUN dnf -y install --setopt=install_weak_deps=False \
+      nodejs \
+      git \
+      curl \
+      jq \
+      rpm-build \
+      rpmdevtools \
+      rpmlint \
+      util-linux \
+      shadow-utils \
+      make \
+      tar \
+      gzip \
+      findutils \
+      diffutils \
+ && dnf clean all
+
+# The nightly job drops to this user rather than building as root (CI
+# containers are root by default). Baked in here rather than created per run.
+RUN useradd --create-home --shell /bin/bash builder
+
+# Sanity check: fail the image build rather than the nightly job if the
+# toolchain ever stops working in this environment.
+RUN runuser -u builder -- bash -c 'rpmbuild --version && rpmspec --version && command -v spectool rpmlint node'