SpringBoot Add support for environment variables with custom prefix

Use case: * Publish a Docker image for a Spring Boot application, for example using com.google.cloud.tools.jib * Use this Docker image in a GitHub Actions workflow * Supply Spring Boot configuration properties using the with: clause in the GitHub Actions workflow

GitHub will pass the contents of the with: clause as environment variables to the Docker image using an INPUT_ prefix, i.e. INPUT_MY_CONFIG_PROPERTY. It would be nice if Spring Boot would support scenarios like these out of the box.

For now, I'm using a somewhat hacky work-around, passing the following custom Environment implementation to SpringApplicationBuilder:

public class MyCustomEnvironment extends StandardEnvironment {
    @Override
    public Map<String, Object> getSystemEnvironment() {
        Map<String, Object> systemEnv = super.getSystemEnvironment();
        Map<String, Object> result = systemEnv;
        if ( systemEnv.containsKey("GITHUB_ACTIONS") ) {
            result = new HashMap<>(systemEnv); 
            result.putAll(systemEnv.entrySet().stream()
                .filter(e->e.getKey().startsWith("INPUT_"))
                .collect(Collectors.toMap(e->e.getKey().replaceFirst("^INPUT_",""), e->e.getValue())));
        }
        return result;
    }
}

Comment From: wilkinsona

Thanks for the suggestion. This is a duplicate of #3450. Could you please add your comment to that issue as the GitHub Actions use case is one that I don't think we've heard before.