make test-docker runs the test suite against a local mock of api.sendgrid.com:

• prism: Prism v2.0.17 mocking oai_stoplight.json. The binary's sha256 is checked in Dockerfile.prism.
• nginx: answers TLS as api.sendgrid.com with the self-signed test certificate in nginx/ and proxies to prism. The certificate and key exist only for this mock.
• helper-runner: the repo's root Dockerfile, which trusts that certificate and runs make test-integ.

oai_stoplight.json is vendored from sendgrid/sendgrid-oai at commit eb7a825bf06dfec7da2622735c5334c0d35da9fa, the same commit the previous remote prism-java.sh script pinned. These files used to be downloaded at test time from that repository, which is no longer publicly readable.