Allow SSL certificate to be passed as a file path

[felixvanoost]

Allows the SSL certificate and private key to be passed to the Docker image as a file path instead of a string (see #1512). This includes the following changes:

• Adds two new environment variables KROKI_SSL_KEY_PATH and KROKI_SSL_CERT_PATH. These are given lower priority than the existing KROKI_SSL_KEY and KROKI_SSL_CERT variables in case of multiple definitions.
• Adds new server tests to check for correct SSL setup in both possible cases (certificate + key provided as strings or as paths).
• Updates the documentation to describe both possible options.

[ggrossetie]

Looks good, thanks 👍🏻