Update tagging logic to make betas more obvious

This makes it much more clear when, for example, our `8` is a beta build, by tagging it explicitly as `8u40-b09` as well:

```console
$ ./generate-stackbrew-library.sh
...

openjdk-8u40-b09-jdk: git://github.com/docker-library/java@9b3b7ed1bdd754fa61b5dd86c0bcd3e3400c6a33 openjdk-8-jdk
openjdk-8u40-b09: git://github.com/docker-library/java@9b3b7ed1bdd754fa61b5dd86c0bcd3e3400c6a33 openjdk-8-jdk
openjdk-8u40-jdk: git://github.com/docker-library/java@9b3b7ed1bdd754fa61b5dd86c0bcd3e3400c6a33 openjdk-8-jdk
openjdk-8u40: git://github.com/docker-library/java@9b3b7ed1bdd754fa61b5dd86c0bcd3e3400c6a33 openjdk-8-jdk
openjdk-8-jdk: git://github.com/docker-library/java@9b3b7ed1bdd754fa61b5dd86c0bcd3e3400c6a33 openjdk-8-jdk
openjdk-8: git://github.com/docker-library/java@9b3b7ed1bdd754fa61b5dd86c0bcd3e3400c6a33 openjdk-8-jdk
8u40-b09-jdk: git://github.com/docker-library/java@9b3b7ed1bdd754fa61b5dd86c0bcd3e3400c6a33 openjdk-8-jdk
8u40-b09: git://github.com/docker-library/java@9b3b7ed1bdd754fa61b5dd86c0bcd3e3400c6a33 openjdk-8-jdk
8u40-jdk: git://github.com/docker-library/java@9b3b7ed1bdd754fa61b5dd86c0bcd3e3400c6a33 openjdk-8-jdk
8u40: git://github.com/docker-library/java@9b3b7ed1bdd754fa61b5dd86c0bcd3e3400c6a33 openjdk-8-jdk
8-jdk: git://github.com/docker-library/java@9b3b7ed1bdd754fa61b5dd86c0bcd3e3400c6a33 openjdk-8-jdk
8: git://github.com/docker-library/java@9b3b7ed1bdd754fa61b5dd86c0bcd3e3400c6a33 openjdk-8-jdk

openjdk-8u40-b09-jre: git://github.com/docker-library/java@bcf0fea5a62cb2a75015e2dc2ce05bbc89afcd7d openjdk-8-jre
openjdk-8u40-jre: git://github.com/docker-library/java@bcf0fea5a62cb2a75015e2dc2ce05bbc89afcd7d openjdk-8-jre
openjdk-8-jre: git://github.com/docker-library/java@bcf0fea5a62cb2a75015e2dc2ce05bbc89afcd7d openjdk-8-jre
8u40-b09-jre: git://github.com/docker-library/java@bcf0fea5a62cb2a75015e2dc2ce05bbc89afcd7d openjdk-8-jre
8u40-jre: git://github.com/docker-library/java@bcf0fea5a62cb2a75015e2dc2ce05bbc89afcd7d openjdk-8-jre
8-jre: git://github.com/docker-library/java@bcf0fea5a62cb2a75015e2dc2ce05bbc89afcd7d openjdk-8-jre
```