Pre-pulls shadowbox container base image before 'docker build' (#669)

* gitignore

* Pre-pulls shadowbox container base image before 'docker build'

* Addresses review comments

src/shadowbox/docker/Dockerfile

@@ -12,13 +12,11 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-# Newer node images have no valid content trust data.
-# Pin the image node:12.16.3-alpine (linux/amd64) by hash.
-# See versions at https://hub.docker.com/_/node?tab=tags&name=alpine
+ARG NODE_IMAGE
 
 # Multi-stage build: use a build image to prevent bloating the shadowbox image with dependencies.
 # Run `yarn` and build inside the container to package the right dependencies for the image.
-FROM node@sha256:12b2154fb459fa5f42c54771524609db041e7ef3465935d0ca82940d2d72669d AS build
+FROM ${NODE_IMAGE} AS build
 
 RUN apk add --no-cache --upgrade bash
 WORKDIR /
@@ -35,7 +33,7 @@ COPY tsconfig.json ./
 RUN ROOT_DIR=/ yarn do shadowbox/server/build
 
 # shadowbox image
-FROM node@sha256:12b2154fb459fa5f42c54771524609db041e7ef3465935d0ca82940d2d72669d
+FROM ${NODE_IMAGE}
 
 # Versions can be found at https://github.com/Jigsaw-Code/outline-ss-server/releases
 ARG SS_VERSION=1.1.4

src/shadowbox/docker/build_action.sh

@@ -16,6 +16,15 @@
 
 export DOCKER_CONTENT_TRUST=${DOCKER_CONTENT_TRUST:-1}
 # Enable Docker BuildKit (https://docs.docker.com/develop/develop-images/build_enhancements)
-# TODO(fortuna): Re-enable after we figure out how to make it work on Travis: https://github.com/moby/buildkit/issues/606
-# export DOCKER_BUILDKIT=1
-docker build --force-rm --build-arg GITHUB_RELEASE="${TRAVIS_TAG:-none}" -t ${SB_IMAGE:-outline/shadowbox} $ROOT_DIR -f src/shadowbox/docker/Dockerfile
+export DOCKER_BUILDKIT=1
+
+# Newer node images have no valid content trust data.
+# Pin the image node:12.16.3-alpine (linux/amd64) by hash.
+# See versions at https://hub.docker.com/_/node?tab=tags&name=alpine
+readonly NODE_IMAGE="node@sha256:12b2154fb459fa5f42c54771524609db041e7ef3465935d0ca82940d2d72669d"
+
+# Doing an explicit `docker pull` of the container base image to work around an issue where
+# Travis fails to pull the base image when using BuildKit. Seems to be related to:
+# https://github.com/moby/buildkit/issues/606 and https://github.com/moby/buildkit/issues/1397
+docker pull "${NODE_IMAGE}"
+docker build --force-rm --build-arg NODE_IMAGE="${NODE_IMAGE}" --build-arg GITHUB_RELEASE="${TRAVIS_TAG:-none}" -t ${SB_IMAGE:-outline/shadowbox} $ROOT_DIR -f src/shadowbox/docker/Dockerfile