Obsolete Java 8 ppa

[pietrobraione]

jpf-star/Dockerfile

Lines 11 to 13 in 4b5c3b2

	add-apt-repository ppa:webupd8team/java -y && \
	apt-get update -y && \
	apt-get install -y oracle-java8-installer

Since April 2019 webupd8 is no longer providing the ppa for Oracle Java 8, because of Oracle's change in license. A possible alternative is the ppa for Trusty of OpenJDK 8 hosted by OpenJDK Builds; the three lines should be changed to:

    add-apt-repository ppa:openjdk-r/ppa -y && \
    apt-get update -y && \
    apt-get install -y openjdk-8-jdk && \

Some other changes are necessary, i.e., deleting the line that removes /var/cache/oracle-jdk8-installer, and setting JAVA_HOME to either /usr/lib/jvm/java-8-openjdk-$ARCH, where ARCH is the architecture.