Missing docker base results in un-actionable error message

[martinda]

Similar to issue #142 , this one is about leaving the Docker Base field blank. When the field is blank, the WAR Download fails with: "This is something wrong with your configuration file. Please fix and try again"

The console shows:

Sending build context to Docker daemon  164.2MB
Error response from daemon: Dockerfile parse error line 1: FROM requires either one or three arguments
2020-08-22 12:14:48.661 ERROR 7194 --- [nio-8080-exec-7] c.o.j.c.j.d.service.PackagerController   : java.io.IOException: Command failed with exit code 1: docker build -t asd .

These errors need to be prevented by forcing a default value or by requiring from the user a proper value. The value should also be checked as early as possible: we do not want to spent time building the WAR only to fail much later in the process.