You want sandboxes that start with your dependencies already installed.
1

Write a Dockerfile

Any base image works. EXPOSE 49983 is refused because the Anona runtime owns that port, and every FROM is pinned by digest at build time.
2

Build it

build_template
3

Start from it

start_from_template
Limits: 20 templates per organization, 2 builds at a time, a build context up to 1 GiB. Details and the SDK route: Templates.
Template builds are verified against a real gateway and BuildKit, but registration against CubeSandbox on AWS has not been run yet.