use custom Dockerfile for Dev Environments image

Signed-off-by: Guillaume Lours <guillaume.lours@docker.com>

.docker/Dockerfile.devenv

@@ -0,0 +1,3 @@
+FROM docker/dev-environments-ruby:stable-1
+RUN gem install bundler jekyll
+CMD ["bundle", "install"]

.docker/config.json

@@ -1,3 +1,3 @@
 {
-    "image": "jekyll/jekyll"
+    "dockerfile": "Dockerfile.devenv"
 }