[build] Make the docker image smaller

It could be made even smaller by splitting Android and Linux
builds, but so far CircleCI is handling it fine.

scripts/ci/Dockerfile

@@ -10,69 +10,78 @@ COPY etc/apt/sources.list /etc/apt
 
 RUN set -eu \
     && apt-get update \
-    && apt-get dist-upgrade -y
+    && apt-get dist-upgrade -y \
+    && apt-get autoremove --purge -y && apt-get clean
 
 # CI requirements
 RUN set -eu && apt-get install -y \
-    ca-certificates \
-    git \
-    gzip \
-    ssh \
-    tar
+      ca-certificates \
+      git \
+      gzip \
+      ssh \
+      tar \
+    && apt-get autoremove --purge -y && apt-get clean
 
 # Base dependencies
 RUN set -eu && apt-get install -y \
-    ccache \
-    clang-8 \
-    clang-format-8 \
-    clang-tidy-8 \
-    cmake \
-    fonts-noto \
-    g++-8 \
-    libc++-8-dev \
-    libc++abi-8-dev \
-    mesa-common-dev \
-    ninja-build \
-    nodejs \
-    npm \
-    pkg-config \
-    python3-pip \
-    software-properties-common \
-    valgrind \
-    xvfb
+      ccache \
+      clang-8 \
+      clang-format-8 \
+      clang-tidy-8 \
+      cmake \
+      fonts-noto \
+      g++-8 \
+      libc++-8-dev \
+      libc++abi-8-dev \
+      mesa-common-dev \
+      ninja-build \
+      nodejs \
+      npm \
+      pkg-config \
+      python3-pip \
+      python3-requests \
+      python3-github \
+      software-properties-common \
+      valgrind \
+      xvfb \
+    && apt-get autoremove --purge -y && apt-get clean
 
 RUN pip3 install cmake-format==0.5.5
 
 # Linux dependencies
 RUN set -eu && apt-get install -y \
-    libcurl4-openssl-dev \
-    libgl1-mesa-dev \
-    libgles2-mesa-dev \
-    libglfw3-dev \
-    libicu-dev \
-    libjpeg-turbo8-dev \
-    libpng-dev \
-    libuv1-dev \
-    zlib1g-dev
+      libcurl4-openssl-dev \
+      libgl1-mesa-dev \
+      libgles2-mesa-dev \
+      libglfw3-dev \
+      libicu-dev \
+      libjpeg-turbo8-dev \
+      libpng-dev \
+      libuv1-dev \
+      zlib1g-dev \
+    && apt-get autoremove --purge -y && apt-get clean
 
 # Qt dependencies
 RUN set -eu && apt-get install -y \
-    qdoc-qt5 \
-    qt5-default
+      qdoc-qt5 \
+      qt5-default \
+    && apt-get autoremove --purge -y && apt-get clean
 
 # Android dependencies
 RUN set -eu && apt-get install -y \
-    coreutils \
-    curl \
-    openjdk-8-jdk-headless \
-    unzip
+      coreutils \
+      curl \
+      openjdk-8-jdk-headless \
+      unzip \
+    && apt-get autoremove --purge -y && apt-get clean
 
 # Install old compilers
 RUN set -eu \
     && apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 1E9377A2BA9EF27F \
     && add-apt-repository "deb http://ppa.launchpad.net/ubuntu-toolchain-r/test/ubuntu xenial main" \
     && add-apt-repository "deb http://us.archive.ubuntu.com/ubuntu/ xenial main universe" \
-    && apt-get install -y g++-5
+    && apt-get install -y g++-5 \
+    && apt-get autoremove --purge -y && apt-get clean
 
 # Install Android NDK
 RUN set -eu \
@@ -107,10 +116,9 @@ RUN set -eu \
 RUN set -eu \
     && echo "deb [signed-by=/usr/share/keyrings/cloud.google.gpg] http://packages.cloud.google.com/apt cloud-sdk main" | tee -a /etc/apt/sources.list.d/google-cloud-sdk.list \
     && curl https://packages.cloud.google.com/apt/doc/apt-key.gpg | apt-key --keyring /usr/share/keyrings/cloud.google.gpg add - \
-    && apt-get update -y && apt-get install google-cloud-sdk -y
+    && apt-get update -y \
+    && apt-get install google-cloud-sdk -y \
+    && apt-get autoremove --purge -y && apt-get clean
 
 # Configure ccache
 RUN set -eu && /usr/sbin/update-ccache-symlinks
-
-# Cleanup
-RUN set -eu && apt-get clean