chore(build): optimize docker image size

[flrgh]

This significantly reduces the docker image size for doorbell, at the cost of a longer build time:

REPOSITORY   TAG         IMAGE ID       CREATED         SIZE
doorbell     main        249339465ea9   4 seconds ago   365MB
doorbell     optimized   9ffc25cad718   4 minutes ago   206MB

Changes:

• Multi-stage: build with openresty:alpine-fat and finalize with openresty:alpine
• Deps: use rusty-cli in place of resty-cli (this allows dropping perl as a runtime dep)