Add Taskcluster config for image building

.taskcluster.yml

@@ -0,0 +1,32 @@
+version: 0
+tasks:
+  - provisionerId: '{{ taskcluster.docker.provisionerId }}'
+    workerType: '{{ taskcluster.docker.workerType }}'
+    extra:
+      github:
+        env: true
+        events:
+          - push
+        branches:
+          - auto
+          - master
+          - try
+    payload:
+      maxRunTime: 3600
+      image: ubuntu@sha256:edf05697d8ea17028a69726b4b450ad48da8b29884cd640fec950c904bfb50ce
+      command:
+        - /usr/bin/env
+        - bash
+        - '--login'
+        - '-c'
+        - >-
+          git clone {{event.head.repo.url}} saltfs
+          && cd saltfs
+          && git config advice.detachedHead false
+          && git checkout {{event.head.sha}}
+          && ./etc/build_image.sh
+    metadata:
+      name: Servo Trusty OCI image builder
+      description: Build Trusty OCI images using Salt for building/testing Servo
+      owner: '{{ event.head.user.email }}'
+      source: '{{ event.head.repo.url }}'

.travis.yml

@@ -59,7 +59,7 @@ matrix:
         - SALT_NODE_ID=servo-linux1
         - SALT_FROM_SCRATCH=true
           # ubuntu:trusty
-          # NOTE: Keep in sync with Dockerfile
+          # NOTE: Keep in sync with Dockerfile, .taskcluster.yml
         - SALT_DOCKER_IMAGE=ubuntu@sha256:084989eb923bd86dbf7e706d464cf3587274a826b484f75b69468c19f8ae354c
       os: linux
       sudo: required

docs/docker.md

@@ -19,6 +19,9 @@ e.g. adding a new dependency.
 
 ## Usage
 
+Docker images are automatically built by Taskcluster on pushes to master,
+as well as when invoked by Homu by pushes to auto and try.
+
 To build manually, run from the saltfs root:
 ```sh
 $ sudo docker build .

etc/build_image.sh

@@ -0,0 +1,19 @@
+#!/usr/bin/env bash
+
+set -o errexit
+set -o nounset
+set -o pipefail
+
+shopt -s nullglob
+
+
+main() {
+    docker build \
+        --force-rm=true \
+        --no-cache=true \
+        --compress=true \
+        ./.
+}
+
+
+main "$@"