ci: add a Docker container for building CMake

Since we're starting with just `clang-tidy`, external deps are not
included right now.

.gitlab/ci/docker/fedora31/Dockerfile

@@ -0,0 +1,5 @@
+FROM fedora:31
+MAINTAINER Ben Boeckel <ben.boeckel@kitware.com>
+
+COPY install_deps.sh /root/install_deps.sh
+RUN sh /root/install_deps.sh

.gitlab/ci/docker/fedora31/install_deps.sh

@@ -0,0 +1,13 @@
+#!/bin/sh
+
+# Install build requirements.
+dnf install -y \
+    openssl-devel
+
+# Install development tools.
+dnf install -y \
+    clang-tools-extra \
+    gcc-c++ \
+    git-core
+
+dnf clean all