Dockerfile: Switch to mage builds, various optimizations

Dockerfile: Switch to mage builds, various optimizations

- Use Alpine for both stages for consistency.
- Switch to mage from go install as dictated in the latest docs.
- Easy switch to building Hugo Extended by directly setting HUGO_BUILD_TAGS flag at image build time: `docker build --build-arg HUGO_BUILD_TAGS=extended .`
- Update to Go 1.13 and Alpine 3.10.
- The only possibly breaking change: Moved the `hugo` binary in the final stage from /hugo to /usr/bin/hugo so one can simply run `hugo` without having to search around for its location.