Support Apple Silicon locally, and test there's no regression

[danyalaytekin]

This PR:

• allows the Docker image to build on Apple Silicon locally by making amd64 explicit (there might be another way to do this than the way I've chosen, so you're welcome to change it)
• adds a test.yml to build the GitHub Action, and then use it to turn the readme into a PDF, including during a pull request. This doesn't test the Apple Silicon compatibility; it just tests that I haven't introduced a regression, and it also might be useful for future pull requests

Testing the build in Apple Silicon on GitHub Actions has not been implemented, because I don't have the time to implement Docker setup in macos-latest, if this is even possible. In any case, it still won't allow the action to be used in GitHub Actions on macos-latest, as observed by #52. This is all just to make local usage easier.

Motivation

1. On Apple Silicon locally, nektos/act fails to run a job which uses markdown-to-pdf, with or without a flag forcing it into amd64 mode:

   $ brew install act
   $ act --platform=linux/amd64
   
   [build.yml/pdf           ]   ❌  Failure - Main BaileyJM02/[email protected]
   [build.yml/pdf           ] The command '/bin/sh -c wget -q -O - https://dl-ssl.google.com/linux/linux_signing_key.pub | apt-key add - &&  ... (etc) ... && rm -rf /var/lib/apt/lists/*' returned a non-zero code: 100

2. markdown-to-pdf also fails to build locally.

[danyalaytekin]

The new workflow hasn't been picked up in this repo yet, but here's an example on my fork:

• https://github.com/danyalaytekin/contribute--markdown-to-pdf/pull/1
• latest run of test.yml: https://github.com/danyalaytekin/contribute--markdown-to-pdf/actions/runs/8549267820

[BaileyJM02]

Appreciate the contribution - thank you!