Due to a bug in GraalVM, let's default to using Mandrel containers for now.

See:

• https://github.com/quarkusio/quarkus/issues/29124
• https://github.com/oracle/graal/issues/5303

@cescoffier @galderz @zakkak I think that's all that's needed to switch to Mandrel container builds.

@galderz I will wait for your green light before merging this.