Various updates for rav1e Docker image

- Build for both the latest stable and the minimum supported rustc
- Include dav1d, cargo-kcov, and rustfmt
- Refactor Dockerfile into stages for caching and image size optimization
- Use variables for versions of packages we install manually
- Build the Docker image when opening an MR in this repository

.gitlab-ci.yml

-Rust rav1e image:
+variables:
+  # Workaround a bug in Gitlab CI
+  DOCKER_TLS_CERTDIR: ""
+
+Docker build:
+  stage: build
+  tags:
+    - dind
+    - privileged
+  variables:
+    IMAGE_TAG: '1.37'
+  services:
+    - docker:dind
+  script:
+    - docker build ./rust-rav1e --build-arg IMAGE_TAG=$IMAGE_TAG
+  only:
+    - merge_requests
+
+Latest stable Rust:
+  stage: build
+  tags:
+    - dind
+    - privileged
+  variables:
+    IMAGE_TAG: '1.37'
+  services:
+    - docker:dind
+  script:
+    - docker build -t $CI_REGISTRY_IMAGE/rust-rav1e:$IMAGE_TAG ./rust-rav1e --build-arg IMAGE_TAG=$IMAGE_TAG
+    - docker login -u gitlab-ci-token -p $CI_JOB_TOKEN $CI_REGISTRY
+    - docker push $CI_REGISTRY_IMAGE/rust-rav1e:$IMAGE_TAG
+  only:
+    - master
+
+Minimum supported Rust:
   stage: build
   tags:
     - dind
@@ -8,7 +42,7 @@ Rust rav1e image:
   services:
     - docker:dind
   script:
-    - docker build -t $CI_REGISTRY_IMAGE/rust-rav1e:$IMAGE_TAG ./rust-rav1e
+    - docker build -t $CI_REGISTRY_IMAGE/rust-rav1e:$IMAGE_TAG ./rust-rav1e --build-arg IMAGE_TAG=$IMAGE_TAG
     - docker login -u gitlab-ci-token -p $CI_JOB_TOKEN $CI_REGISTRY
     - docker push $CI_REGISTRY_IMAGE/rust-rav1e:$IMAGE_TAG
   only:

rust-rav1e/Dockerfile

-FROM rust:1.36
+ARG IMAGE_TAG
+
+FROM rust:${IMAGE_TAG}-buster
+
+ARG NASM_VERSION=2.14
+ARG KCOV_VERSION=36
+ARG AOM_VERSION=v1.0.0-errata1
+ARG DAV1D_VERSION=0.4.0
 
 RUN apt-get update -qq && \
-    apt-get install -qqy \
-        autoconf \
-        automake \
-        autopoint \
-        gettext \
-        libtool-bin \
-        bzip2 \
-        flex \
-        make \
-        gcc \
-        g++ \
-        g++-multilib \
-        git \
-        gperf \
-        patch \
-        pkg-config \
-        sed \
-        unzip \
-        wget \
-        lzip \
-        xz-utils \
-        cmake \
-        ninja-build \
-        libclang-dev \
-        clang \
-        libelf-dev \
-        libdw-dev \
-        libiberty-dev \
-        binutils-dev
-RUN rustup component add clippy && \
-    curl -L https://download.videolan.org/contrib/nasm/nasm-2.14.tar.gz | tar xvz && \
-    cd nasm-2.14 && \
+    apt-get install -qqy --no-install-recommends \
+    autoconf \
+    automake \
+    autopoint \
+    gettext \
+    libtool-bin \
+    bzip2 \
+    flex \
+    make \
+    gcc \
+    g++ \
+    g++-multilib \
+    git \
+    gperf \
+    patch \
+    pkg-config \
+    sed \
+    unzip \
+    wget \
+    lzip \
+    xz-utils \
+    cmake \
+    ninja-build \
+    libclang-dev \
+    clang \
+    libelf-dev \
+    libdw-dev \
+    libiberty-dev \
+    binutils-dev \
+    meson && \
+    rm -rf /var/lib/apt/lists/*
+RUN curl -L "https://download.videolan.org/contrib/nasm/nasm-$NASM_VERSION.tar.gz" | tar xvz && \
+    cd "nasm-$NASM_VERSION" && \
     ./configure && make -j2 && make install && \
     cd .. && \
-    curl -L https://github.com/SimonKagstrom/kcov/archive/v36.tar.gz | tar xvz && \
-    cd kcov-36 && \
+    rm -rf "nasm-$NASM_VERSION"
+RUN curl -L "https://github.com/SimonKagstrom/kcov/archive/v$KCOV_VERSION.tar.gz" | tar xvz && \
+    cd "kcov-$KCOV_VERSION" && \
     mkdir .build && cd .build && \
     cmake -GNinja .. && \
     ninja && ninja install && \
     cd ../.. && \
-    git clone --depth 1 -b v1.0.0-errata1 https://aomedia.googlesource.com/aom && \
+    rm -rf "kcov-$KCOV_VERSION"
+RUN git clone --depth 1 -b "$AOM_VERSION" https://aomedia.googlesource.com/aom && \
     cd aom && \
     mkdir -p .build && cd .build && \
     cmake -GNinja .. \
-        -DCMAKE_BUILD_TYPE=Release \
-        -DENABLE_TESTS=0 \
-        -DENABLE_DOCS=0 \
-        -DCONFIG_LOWBITDEPTH=1 \
-        -DCMAKE_INSTALL_PREFIX=/usr \
-        -DCONFIG_PIC=1 && \
-    ninja && ninja install
+    -DCMAKE_BUILD_TYPE=Release \
+    -DENABLE_TESTS=0 \
+    -DENABLE_DOCS=0 \
+    -DCONFIG_LOWBITDEPTH=1 \
+    -DCMAKE_INSTALL_PREFIX=/usr \
+    -DCONFIG_PIC=1 && \
+    ninja && ninja install && \
+    cd ../.. && \
+    rm -rf aom
+RUN curl -L "https://code.videolan.org/videolan/dav1d/-/archive/$DAV1D_VERSION/dav1d-$DAV1D_VERSION.tar.gz" | tar xvz && \
+    cd "dav1d-$DAV1D_VERSION" && \
+    meson build --buildtype release && \
+    ninja -C build install && \
+    cd ../.. && \
+    rm -rf "dav1d-$DAV1D_VERSION"
+RUN rustup component add clippy rustfmt
+RUN cargo install cargo-kcov