docker-compose environment will cover variables in jar, how are docker environment variable and variable in jar mapped?

For example, set environment: EUREKA_PORT=8762 in docker-compose.yml.

And the variable ${eureka.port=8761} in jar will not effective.

Why environment: EUREKA_PORT=8762 will cover ${eureka.port=8761}?

How are docker environment variable and variable in jar mapped?