Dockerfile WIP for testing

Dockerfile

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+FROM ubuntu:xenial
+LABEL maintainer="servo-dockerfile-maintainers@mozilla.com"
+
+# Note: This Dockerfile sets up the test environment for running Servo. It
+# does NOT actually perform any build steps by default -- the build steps are
+# listed in etc/ci/buildbot_steps.yml.
+
+# Rust installation from https://hub.docker.com/r/jimmycuadra/rust/
+ENV USER root
+ENV RUST_VERSION=1.18.0
+
+RUN apt-get update && \
+  DEBIAN_FRONTEND=noninteractive apt-get install -y --no-install-recommends \
+    build-essential \
+    ca-certificates \
+    curl \
+    git \
+    libssl-dev \
+    pkg-config && \
+  curl -sO https://static.rust-lang.org/dist/rust-$RUST_VERSION-x86_64-unknown-linux-gnu.tar.gz && \
+  tar -xzf rust-$RUST_VERSION-x86_64-unknown-linux-gnu.tar.gz && \
+  ./rust-$RUST_VERSION-x86_64-unknown-linux-gnu/install.sh --without=rust-docs && \
+  DEBIAN_FRONTEND=noninteractive apt-get remove --purge -y curl && \
+  DEBIAN_FRONTEND=noninteractive apt-get autoremove -y && \
+  rm -rf \
+    rust-$RUST_VERSION-x86_64-unknown-linux-gnu \
+    rust-$RUST_VERSION-x86_64-unknown-linux-gnu.tar.gz \
+    /var/lib/apt/lists/* \
+    /tmp/* \
+    /var/tmp/* && \
+  mkdir /source
+
+# Servo's apt packages & Python salt state
+RUN apt-get update && apt-get install -y \
+  xvfb \
+  python \
+  python3 \
+  python-pip
+
+RUN pip install virtualenv
+
+# Servo's common salt state
+
+RUN useradd servo
+
+# TODO: cross: Servo's servo-build-depenencies salt state
+# TODO: cross: Servo's servo-build-dependencies.android
+# TODO: cross: Servo's servo-build-dependencies.arm
+# TODO: Servo's xvfb state
+# RUN wget https://raw.githubusercontent.com/servo/saltfs/master/xvfb/xvfb.conf /etc/init/xvfb.conf
+# TODO: the builder-specific bits from saltfs/buildbot/master/files/config/environments.py
+
+# etc/ci/buildbot_steps.yml, as hardcoded in salt factories.py, contains
+# steps to run
+
+