It sounds like you'd want to lock to a specific version of the java buildpack, before 94276a, which dokku supports, according to the docs provided in the GP comment:
> You may also wish to use a specific version of a buildpack, which is also simple
# using v87 of the nodejs buildpack
# replace APP with the name of your application
dokku config:set APP BUILDPACK_URL=https://github.com/heroku/heroku-buildpack-nodejs#v87