• (docker) Docker image based on Debian buster
• (docker) Replace ssmtp with msmtp
• Add missing libsdl2-2.0-0:i386
• (docker) GSLT_FILE, STEAM_LOGIN_FILE, STEAM_PASSWORD_FILE and API_AUTHORIZATION_KEY_FILE can be used to fill in the value from a file, especially for Docker's secrets feature.
• (docker) SSMTP_* env vars for Docker removed. See Environment variables section.