prebuild dev container

[caindy]

Addresses #18

• Corporate repository proxies are supported by editing devcontainer.json, see README
• The github-metadata plugin used by github-pages makes a remote request, so we still need to support self-signed certs in the local dev environment. This is supported by placing the CA root PEM file in .devcontainers/root.pem. This will set an OpenSSL environment variable that is respected by the Ruby library making the request.

[caindy]

Ok, I think I have all the issues sorted, assuming you can pull from ghcr.io.