packer/alpine-linux update to 3.16

This updates the packer/alpine-linux toolchain for 3.16, plus some minor improvements.

The work is done to allow to build more up-to date (frr 8.2.2) versions of the FRRouting marketplace appliance (appliances/frr.gns3a) which as far as I can see seem to be built from this toolchain.

I tested the manual import of a locally-built image with the same settings of the frr.gns3a appliance, and the image booted without issues. I only performed basic configuration tests on it.

I did not test any of the other images buildable by the toolchain. I did not see clear traces of them in the marketplace, so I guessed they were probably retired, or used only as intermediate products or local images.

In a not entirely related issue (#697) I was suggested to provide a reference image file to ease the team's work, and encouraged to provide an update .gns3a template. Unfortunately I do not have access to a long term cloud storage solution suitable for sharing, I could provide only ephemeral links. As such, it seems pointless to provide a .gns3a update if I cannot provide proper links for the images. I would appreciate your help in:

Building an image following the documented procedure: packer build -var-file frr.json alpine.json
Uploading the image to a proper hosting solution
Updating appliances/frr.gns3a to include the new image/version 8.2.2

Sorry for the additional work, I hope the contribution is welcome anyway.

GNS3 / gns3-registry

packer/alpine-linux update to 3.16 #701