Update Alpine base, fix Jamulus version download

[Procsiab]

Greetings grundic, I am opening this PR to let you know that with the proposed change to the Dockerfile it is possible to successfully build a container image for the latest "stable" version of Jamulus. I also used an environment variable to address the specific version tag, for managing builds for specific versions more easily.

If for whaterver reason this PR does not fit within your use case, please close it so that it won't add noise. Regards!