Use docker volume for jekyll dependencies

Using a volume allows "caching" the bundled dependencies for jekyll.
Created a "fresh" target for also removing volumes.

.travis.yml

@@ -1,6 +1,6 @@
 ---
 env:
-  - IMAGE="yadm/jekyll:latest"
+  - IMAGE="yadm/jekyll:2019-10-17"
 language: minimal
 services:
   - docker

Makefile

@@ -1,4 +1,5 @@
 IGNORED = .jekyll-metadata Gemfile.lock _site
+VOLUME_ARG =
 
 .PHONY: all
 all:
@@ -37,6 +38,9 @@ usage:
 	@echo '  make clean'
 	@echo '    - Remove previously built data and any jekyll containers.'
 	@echo
+	@echo '  make fresh'
+	@echo '    - Like "make clean", but also removes the docker volumes.'
+	@echo
 
 .PHONY: test
 test: require-docker-compose clean
@@ -62,12 +66,16 @@ restart: require-docker-compose
 
 .PHONY: down
 down: require-docker-compose
-	docker-compose down --remove-orphans
+	docker-compose down --remove-orphans ${VOLUME_ARG}
 
 .PHONY: clean
 clean: down
 	rm -rf ${IGNORED}
 
+.PHONY: fresh
+fresh: VOLUME_ARG = -v
+fresh: clean
+
 .PHONY: require-docker-compose
 require-docker-compose: require-docker
 	@if ! command -v "docker-compose" >/dev/null 2>&1; then \

docker-compose.yml

@@ -2,9 +2,12 @@
 version: '3'
 services:
   website:
-    image: yadm/jekyll:latest
+    image: yadm/jekyll:2019-10-17
     ports:
       - 4000:4000
     volumes:
       - .:/srv/jekyll
+      - jekyll-bundle:/usr/local/bundle
     command: jekyll serve --config _config.yml,_dev.yml -t --watch --incremental
+volumes:
+  jekyll-bundle: