Java heap constraints in docker-compose

docker-compose.yml

version: '2'
services:

  test-java-params-none:
    image: iron/java:1.8
    command: sh -c "         java -XX:+PrintFlagsFinal -version|grep -i heapsize|egrep 'Initial|Max'"

  test-java-params-cmd-line:
    image: iron/java:1.8
    command: sh -c "sleep 2; java -Xms64m -Xmx128m -XX:+PrintFlagsFinal -version|grep -i heapsize|egrep 'Initial|Max'"

  test-java-params-from-dot-env:
    image: iron/java:1.8
    command: sh -c "sleep 4; java ${MEM_OPTS} -XX:+PrintFlagsFinal -version|grep -i heapsize|egrep 'Initial|Max'"

  test-java-params-from-shell-env:
    image: iron/java:1.8
    environment:
      - _JAVA_OPTIONS=-Xms64m -Xmx160m
    command: sh -c "sleep 6; java  -XX:+PrintFlagsFinal -version|grep -i heapsize|egrep 'Initial|Max'"

  test-java-params-from-env-file:
    image: iron/java:1.8
    env_file:
      - tight-mem.env
    command: sh -c "sleep 8; java  -XX:+PrintFlagsFinal -version|grep -i heapsize|egrep 'Initial|Max'"

tight-mem.env

_JAVA_OPTIONS=-Xms12m -Xmx28m