Add an alpine 3.9 minimal agent image

[KierranM]

Adds a minimal agent image based on the openjdk alpine image.

The agent should be equivalent to the ubuntu minimal agent image.

Currently it:

• Creates a new user/goup called buildagent
• Installs sudo and bash
• Copies the agent distribution

[kir]

Hello @KierranM ,

Thanks for the contribution, but I'm afraid the current policy is to use amazon-corretto as the base JDK for our images, and we don't have plans for providing official alpine-based images yet.

Thanks,