Add variant based on Azul Zulu 11 Alpine image.

[reitzig]

There is currently no image based on JDK 11 and Alpine. This PR closes that gap.

[reitzig]

Not sure where the tags on Docker Hub come from. Would propose the following:

• 3.6.1-jdk-11-alpine, 3.6-jdk-11-alpine, 3-jdk-11-alpine
• 3.6.1-azulzulu-alpine, 3.6-azulzulu-alpine, 3-azulzulu-alpine
• 3.6.1-azulzulu-11-alpine, 3.6-azulzulu-11-alpine, 3-azulzulu-11-alpine

[carlossg]

azul based images can't be pushed to https://hub.docker.com/_/maven because azul images are not Docker official images.
I can merge but they won't be built

[reitzig]

That's unfortunate. :/

I've inquired upstream as to why there still is no Alpine-based OpenJDK image.

[reitzig]

Side note: In our production thingy, we need to install gcompat. Is that a special need or would it always be needed?

[reitzig]

Since it seems OpenJDK 11 is in the Alpine repos after all, I created #121 .