Enviroment vars in functions postgressql

Question

I have this function in postgresSQL and I need some help to take them from enviroment.

Adding Vars in DockerFile

CREATE OR REPLACE FUNCTION prc_get_connstr()
RETURNS TEXT AS
$$
DECLARE
  res TEXT;
  SRC_ADDR varchar(50) := `echo ${SRC_ADDR_CRD:-noaddr}`;
  SRC_PORT varchar(50) := `echo ${SRC_PORT_CDR:-noport}`;
  SRC_DB varchar(50) := `echo ${SRC_DB_CDR:-nodb}`;
  SRC_USR varchar(50) := `echo ${SRC_USR_CDR:-nouser}`;
  SRC_PASS varchar(50) := `echo ${SRC_PASS_CDR:-nopass}`;
BEGIN
  RETURN 'dbname=SRC_DB host=SRC_ADDR port=SRC_PORT user=SRC_USR password=SRC_PASS connect_timeout=5';
END;
$$
LANGUAGE 'plpgsql';

When I deploy it doesn't work

You are now connected to database "cdr" as user "postgres".
CREATE EXTENSION
CREATE FUNCTION
psql:/docker-entrypoint-initdb.d/2_cdr_trigger.sql:31: ERROR:  syntax error at or near "$"
LINE 6:   SRC_ADDR varchar(50) := `echo ${SRC_ADDR_CRD:-noaddr}`;

What i have to do to declare de enviroment vars.