For those who consider https://12factor.net/config and work with Spring or Spring Boot, I created a lightweight library that provides a property source for Spring.
Stackoverflow discussion: https://stackoverflow.com/questions/58549361/using-dotenv-files-with-spring-boot
Github repo: https://github.com/paulschwarz/spring-dotenv
Maven artifact: https://search.maven.org/artifact/me.paulschwarz/spring-dotenv