ci: add an image for Debian 10

This image is primarily for the include-what-you-use build.

.gitlab/ci/docker/debian10/Dockerfile

@@ -0,0 +1,15 @@
+FROM debian:10 as iwyu-build
+MAINTAINER Ben Boeckel <ben.boeckel@kitware.com>
+
+COPY install_iwyu.sh /root/install_iwyu.sh
+RUN sh /root/install_iwyu.sh
+
+FROM debian:10
+MAINTAINER Ben Boeckel <ben.boeckel@kitware.com>
+
+COPY install_deps.sh /root/install_deps.sh
+RUN sh /root/install_deps.sh
+
+COPY --from=iwyu-build /root/iwyu.tar.gz /root/iwyu.tar.gz
+RUN tar -C / -xf /root/iwyu.tar.gz
+RUN ln -s /usr/lib/llvm-6.0/bin/include-what-you-use /usr/bin/include-what-you-use-6.0

.gitlab/ci/docker/debian10/install_deps.sh

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+#!/bin/sh
+
+set -e
+
+apt-get update
+
+# Install build requirements.
+apt-get install -y \
+    libssl-dev
+
+# Install development tools.
+apt-get install -y \
+    g++ \
+    curl \
+    git
+
+# Install iwyu runtime deps.
+apt-get install -y \
+    clang-6.0 \
+    libncurses6
+
+apt-get clean

.gitlab/ci/docker/debian10/install_iwyu.sh

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+#!/bin/sh
+
+set -e
+
+# Install development tools.
+apt-get update
+apt-get install -y \
+    clang-6.0 \
+    libclang-6.0-dev \
+    llvm-6.0-dev \
+    libz-dev \
+    g++ \
+    cmake \
+    ninja-build \
+    git
+
+cd /root
+git clone "https://github.com/include-what-you-use/include-what-you-use.git"
+cd include-what-you-use
+readonly llvm_version="$( clang-6.0 --version | head -n1 | cut -d' ' -f3 | cut -d. -f-2 )"
+git checkout "clang_$llvm_version"
+mkdir build
+cd build
+
+cmake -GNinja \
+    -DCMAKE_BUILD_TYPE=Release \
+    "-DCMAKE_INSTALL_PREFIX=/usr/lib/llvm-$llvm_version" \
+    "-DIWYU_LLVM_ROOT_PATH=/usr/lib/llvm-$llvm_version" \
+    ..
+ninja
+DESTDIR=/root/iwyu-destdir ninja install
+tar -C /root/iwyu-destdir -cf /root/iwyu.tar.gz .