Dockerfile: add or document an alpine setup

[a-gave]

document an alpine setup to self build smaller imagebuilder containers

[a-gave]

I saw there was a little interest on this: #8

It seems convenient to have also alpine based images to reduce the size of imagebuilders and sdks, eg:

ghcr.io/openwrt/imagebuilder-alpine  ath79-generic-v24.10.1        460321c3b32e  7 weeks ago        1.56 GB
ghcr.io/openwrt/imagebuilder     ath79-generic-v24.10.1   6b78c2158b99   8 weeks ago     2.07GB

and for faster pulls from registries.

Maybe the list of requirements installed by apk could be reviewed
Smoke tests from the ci seem to work correctly.

ping @aparcar who reviewed #8

[a-gave]

fix: move docker args and envs after the installation of system requirements.

[aparcar]

@ynezz what do you think?

[a-gave]

fix: set default distros when no input from workflow dispatch is provided
fix: run rootfs builds once since they are then based on scratch

[aparcar]

Thanks for your work, some minor changes please

[a-gave]

Thanks for reviewing. Submitted the requested changes.

[a-gave]

fd93873 adds the missing path to Dockerfile also to the step Push.

I loaded some images based on alpine at https://hub.docker.com/r/agave0/openwrt-imagebuilder
There are still some minor things to clean, like it is missing /builder/tmp and ib is using as fallback /tmp