Have you verified the war (/app/build/libs/sampleWeb-0.0.1-SNAPSHOT.war) file mentioned in the Dockerfile is created in your project target place prior to run the app ?

This could also be one of the possible reason when the project ain't built (mvn build/package) and run the application before build either from terminal or editor, in that case docker searches in the target place for .war/.jar package which obviously isn't there and throws an error.