Update Dockerfile to use Alpine Linux

[jessie-murray]

• Change base image to Alpine 3.11.3
• Use multi-stage build (reducing size from 276 MB to 9.5 MB)
• Change Makefile to build with -O3 instead of -O0 -ggdb

[nicolasff]

Hi @jessie-murray, thanks for the PR! I didn't expect multi-stage builds to make such a difference, this is great.