Merge package installation into a single layer

Currently, CI is failing weirdly because of ubuntu repos returning 404.
I suspect that is because the package index layer, `apt-get update`, is
a cache-hit but the next layer that installs packages are not. This
causes `apt` to work with outdated indices.

Further, it's just good container hygiene to keep those in a single
layer to make sure package index doesn't end up in a layer of the image.

tests/docker/rockylinux.Dockerfile

@@ -1,13 +1,12 @@
 FROM rockylinux:9
 
 # Enable EPEL
-RUN dnf update -y
-RUN dnf install -y 'dnf-command(config-manager)'
-RUN dnf config-manager --set-enabled crb -y
-RUN dnf install epel-release -y
-
 # Install dependencies
-RUN dnf install -y \
+RUN dnf update -y \
+    dnf install -y 'dnf-command(config-manager)' \
+    dnf config-manager --set-enabled crb -y \
+    dnf install epel-release -y \
+    dnf install -y \
         python3.11 \
         python3-pip \
         pkgconf-pkg-config \
@@ -20,8 +19,8 @@ RUN dnf install -y \
         expected-devel \
         gtest-devel \
         fmt-devel \
-        cli11-devel
-RUN dnf clean all
+        cli11-devel \
+    dnf clean all
 RUN update-alternatives --install /usr/local/bin/python python /usr/bin/python3.11 10
 # Install meson from pip
 RUN python3 -m pip install -U meson==0.64.1

tests/docker/ubuntu.Dockerfile

@@ -1,8 +1,8 @@
 FROM ubuntu:22.04
 
 # Install dependencies
-RUN apt-get update
-RUN apt-get install -y \
+RUN apt-get update && \
+    apt-get install -y \
         python3.11 \
         python3-pip \
         pkg-config \
@@ -15,8 +15,8 @@ RUN apt-get install -y \
         libexpected-dev \
         libgtest-dev \
         libfmt-dev \
-        libcli11-dev
-RUN apt-get clean
+        libcli11-dev && \
+    apt-get clean
 RUN update-alternatives --install /usr/local/bin/python python /usr/bin/python3.11 10
 # Install meson from pip
 RUN python3 -m pip install -U meson==0.64.1