Use gohugoio org inread of spf13 during source downloading

See merge request pages/hugo!91

Signed-off-by: Takuya Noguchi <takninnovationresearch@gmail.com>

Workaround for segmentation faults in hugo_extended registry image

See merge request pages/hugo!86

- Run muslstack on hugo executable to extend thread stack size to 8MB
- Introduce multi-stage builds to reduce image size further
- Use alpine:image for the final image

muslstack utility:
https://github.com/yaegashi/muslstack

muslstack test cases using Hugo extended version:
https://github.com/yaegashi/muslstack/tree/master/tests/hugo

Build normal and extended versions for the registry images

See merge request pages/hugo!83

Closes pages/hugo#29