Dockerfile does not work without setting workdir even if all file references are absolute

Dockerfile-broken

FROM dockerfile/java

# copy the locally built fat-jar to the image
ADD app.jar /

EXPOSE 42042

# run the server when a container based on this image is being run
ENTRYPOINT [ "java", "-verbose",  "-jar", "/app.jar" ]

### hangs after printing "[Opened /usr/lib/jvm/java-7-oracle/jre/lib/rt.jar]" 
### note that this is only because running the jar with "-verbose" 
### otherwise there would be no output

Dockerfile-working

FROM dockerfile/java

WORKDIR /

# copy the locally built fat-jar to the image
ADD app.jar /

EXPOSE 42042

# run the server when a container based on this image is being run
ENTRYPOINT [ "java", "-verbose",  "-jar", "/app.jar" ]

### CORRECTLY RUNS THE JAR