Update Dockerfile to a multi-stage build

- Hugo container is based on SCRATCH to further reduce the footprint
  and the vulnerability surface
- Update Alpine image to 3.7 in the build container
- Update Go Lang to 1.10 in the build container
- Add .dockerignore file per the Docker best practices

Closes #4154, #4155, #4157

.dockerignore

@@ -0,0 +1,8 @@
+*.md
+*.log
+*.txt
+.git
+.github
+.circleci
+docs
+examples