Change to Spring Placeholder Syntax

Let's prioritize running within the project for now.
Due to a difference between Spring and Docker's interpolation
syntax, the projects will favor Spring syntax so that
both the application and its tests can run successfully.

servlet/spring-boot/java/saml2/identity-provider/src/main/resources/docker/compose.yml

@@ -5,15 +5,15 @@ services:
       - ./metadata/authsources.php:/var/www/simplesamlphp/config/authsources.php
       - ./metadata/one-relyingparties.php:/var/www/simplesamlphp/metadata/saml20-sp-remote.php
     environment:
-      - PORT=${SERVER_PORT:-8080}
+      - PORT=${SERVER_PORT:8080}
 
   idp-two.7f000001.nip.io:
     image: kristophjunge/test-saml-idp:1.15
     volumes:
       - ./metadata/authsources.php:/var/www/simplesamlphp/config/authsources.php
       - ./metadata/two-relyingparties.php:/var/www/simplesamlphp/metadata/saml20-sp-remote.php
     environment:
-      - PORT=${SERVER_PORT:-8080}
+      - PORT=${SERVER_PORT:8080}
 
   nginx:
     image: nginx:stable