this.connection = `DriverManager.getConnection("jdbc:postgresql://<myip>:5432/databasename", "usr", "password");`

Connection refused.  Check that the hostname and port are correct and that the postmaster is accepting TCP/IP connections.

FROM java:8
ADD VaultServer /
EXPOSE 3971
EXPOSE 3972
ENTRYPOINT ["java", "-jar", "VaultServer.jar"]