Remove Oracle Docker image

[jdubois]

Unfortunately it is not possible to use a public Docker image for the Oracle database:

• The one we had been using, sath89/oracle-12c has been removed
• As we can see on Did this get taken off the Docker repos? wnameless/docker-oracle-xe-11g#118 and on Open Source frameworks need a useable Oracle XE CI Image oracle/docker-images#1156 Oracle is sending DMCA requests and asking everyone to remove those images

As I don't want us (or our users) to get into trouble, we need to remove this. That will be less users for Oracle!