Cache Docker Linux build on Travis, fixes #40

Inspired by:
travis-ci/travis-ci#5358 (comment)
and
http://rundef.com/fast-travis-ci-docker-build

.travis.yml

@@ -5,9 +5,29 @@ language: generic
 services:
   - docker
 
+env:
+  global:
+    - CACHE_DIR=$HOME/.cache
+    - DOCKER_CACHE_DIR=$CACHE_DIR/docker
+
+cache:
+  directories:
+    - $DOCKER_CACHE_DIR
+
 before_install:
+# load cached Docker images
+  - if [[ -d $DOCKER_CACHE_DIR ]]; then ls $DOCKER_CACHE_DIR/*.tar.gz | xargs -I {file} sh -c "zcat {file} | docker load"; fi
+
+install:
   - docker build --tag=etheroll .
 
 script:
   - docker run -e ETHERSCAN_API_KEY=$ETHERSCAN_API_KEY etheroll
     /bin/sh -c '. venv/bin/activate && make test'
+
+before_cache:
+  # save tagged Docker images
+  # https://github.com/travis-ci/travis-ci/issues/5358#issuecomment-248915326
+  - >
+    mkdir -p $DOCKER_CACHE_DIR && docker images -a --filter='dangling=false' --format '{{.Repository}}:{{.Tag}} {{.ID}}'
+    | xargs -n 2 -t sh -c 'test -e $DOCKER_CACHE_DIR/$1.tar.gz || docker save $0 | gzip -2 > $DOCKER_CACHE_DIR/$1.tar.gz'

CHANGELOG.md

@@ -4,6 +4,7 @@
 
   - Setup continuous integration, refs #18
   - Dockerized project, refs #19
+  - Caching Docker Linux build on Travis, refs #40
 
 
 ## [v20180405]