The Docker image should use a specific release of scitokens-java

[duncan-brown]

Currently the image grabs some parts from the 1.2-a pre-release and builds other parts from source. Once @jjg-123 has released 1.2 that contains the files

scitokens-server.war
scitokens-util.jar
scitokens-cli.jar
server-config.xml
scitokens-client.war
sample-client-config.xml

then the Dockerfile should be updated with a ARG for the release and download these specific files.