Docker: add docs, bump all digests

Use the latest ubuntu:trusty and ubuntu:xenial.
Also, update the Dockerfile so it can be built without any build args,
and add some documentatation about how to do so.

.travis.yml

@@ -58,16 +58,17 @@ matrix:
     - env:
         - SALT_NODE_ID=servo-linux1
         - SALT_FROM_SCRATCH=true
-          # ubuntu/14.04
-        - SALT_DOCKER_IMAGE=ubuntu@sha256:edf05697d8ea17028a69726b4b450ad48da8b29884cd640fec950c904bfb50ce
+          # ubuntu:trusty
+          # NOTE: Keep in sync with Dockerfile
+        - SALT_DOCKER_IMAGE=ubuntu@sha256:084989eb923bd86dbf7e706d464cf3587274a826b484f75b69468c19f8ae354c
       os: linux
       sudo: required
       dist: trusty
     - env:
         - SALT_NODE_ID=servo-linux1
         - SALT_FROM_SCRATCH=true
-          # ubuntu/16.04
-        - SALT_DOCKER_IMAGE=ubuntu@sha256:f3a61450ae43896c4332bda5e78b453f4a93179045f20c8181043b26b5e79028
+          # ubuntu:xenial
+        - SALT_DOCKER_IMAGE=ubuntu@sha256:ec0e4e8bf2c1178e025099eed57c566959bb408c6b478c284c1683bc4298b683
       os: linux
       sudo: required
       dist: trusty

Dockerfile

@@ -1,7 +1,9 @@
-# Ubuntu Xenial
-FROM ubuntu@sha256:a0ee7647e24c8494f1cf6b94f1a3cd127f423268293c25d924fbe18fd82db5a4
+# ubuntu:trusty
+# NOTE: Keep in sync with .travis.yml
+FROM ubuntu@sha256:084989eb923bd86dbf7e706d464cf3587274a826b484f75b69468c19f8ae354c
 
 ARG SALT_ROOT=/tmp/salt-bootstrap
+ARG SALT_NODE_ID=servo-linux1
 
 COPY ./ "${SALT_ROOT}"
 
@@ -16,7 +18,6 @@ RUN : \
         --id="${SALT_NODE_ID}" \
         --config-dir="${SALT_ROOT}/.travis" \
         --file-root="${SALT_ROOT}" \
-        --log-level=warning \
         --retcode-passthrough \
         --force-color \
         state.apply common,servo-build-dependencies \

docs/docker.md

@@ -0,0 +1,28 @@
+# Docker usage
+
+## Overview
+
+As part of a move to [TaskCluster](https://docs.taskcluster.net/),
+we are moving Linux builds from VMs
+to [Docker](https://www.docker.com/) containers.
+The Dockerfile in this repository will be used to create container images
+that are suitable for running Servo builds.
+To ease the transition, we will reuse our [Salt](https://saltstack.com/)
+rules for provisioning in containers.
+
+In the future, the Dockerfile and supporting files will be moved into the main
+Servo repository.
+This will allow our decision task to rebuild our Docker
+images when there are any changes,
+to enable making builder configuration changes concurrently with code changes,
+e.g. adding a new dependency.
+
+## Usage
+
+To build manually, run from the saltfs root:
+```sh
+$ sudo docker build .
+```
+
+Currently, this creates an image capable only of building Servo itself,
+not testing (no Xvfb) or cross-compiling (Android, ARM, etc.) yet.