Move etc/ci/taskcluster one level up

etc/taskcluster/docker/base.dockerfile

@@ -0,0 +1,23 @@
+FROM ubuntu:bionic-20180821
+
+ENV \
+    #
+    # Some APT packages like 'tzdata' wait for user input on install by default.
+    # https://stackoverflow.com/questions/44331836/apt-get-install-tzdata-noninteractive
+    DEBIAN_FRONTEND=noninteractive
+
+RUN \
+    apt-get update -q && \
+    apt-get install -qy --no-install-recommends \
+        #
+        # Cloning the repository
+        git \
+        ca-certificates \
+        #
+        # Running mach
+        python2.7 \
+        virtualenv \
+        #
+        # Installing rustup and sccache (build dockerfile) or fetching build artifacts (run tasks)
+        curl
+

etc/taskcluster/docker/build.dockerfile

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+% include base.dockerfile
+
+RUN \
+    apt-get install -qy --no-install-recommends \
+        #
+        # Multiple C/C++ dependencies built from source
+        g++ \
+        make \
+        cmake \
+        #
+        # ANGLE
+        xorg-dev \
+        #
+        # mozjs (SpiderMonkey)
+        autoconf2.13 \
+        #
+        # Bindgen (for SpiderMonkey bindings)
+        clang \
+        #
+        # GStreamer
+        libgstreamer-plugins-bad1.0-dev \
+        #
+        # OpenSSL
+        libssl1.0-dev \
+        #
+        # blurz
+        libdbus-1-dev \
+        #
+        # Skia
+        libglu1-mesa-dev \
+        libbz2-dev \
+        #
+        #
+    && \
+    #
+    #
+    curl https://sh.rustup.rs -sSf | sh -s -- --default-toolchain none -y && \
+    #
+    #
+    curl -sSfL \
+        https://github.com/mozilla/sccache/releases/download/0.2.7/sccache-0.2.7-x86_64-unknown-linux-musl.tar.gz \
+        | tar -xz --strip-components=1 -C /usr/local/bin/ \
+            sccache-0.2.7-x86_64-unknown-linux-musl/sccache

etc/taskcluster/docker/run.dockerfile

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+% include base.dockerfile
+
+# Servo’s runtime dependencies
+RUN apt-get install -qy --no-install-recommends \
+    libgl1 \
+    libssl1.0.0 \
+    libdbus-1-3 \
+    libgstreamer-plugins-bad1.0-0 \
+    gstreamer1.0-plugins-good