ARM support in Docker build

[mojotalantikite]

The current Docker container is built off of openjdk:12-alpine, which unfortunately doesn't have ARM support. Looking at the openjdk Docker Hub page, it doesn't look like any of the Alpine image tags have ARM support in general. With the new Apple M1 silicon this will cause issues if you're trying to build locally to test openapi generation.

I also noticed that in #27 the Dart SDK generation was having issues with musl in the Alpine image. Between these two issues it might make sense to move to another base image altogether.

I did some testing with the Debian openjdk:16-slim-buster container and I was able to generate libraries for Typescript just fine.

I'll put together a PR in a few for others to take a look at in case there is interest in this. One thing to note is the container size is a lot bigger.