add alpine base image

[Gfeuillen]

Simply adding an alpine based image using multi-step build.

[timoschlueter]

Great idea! I am actually wondering if we should make this the default image since it is much smaller.
What do you think? :)

[Gfeuillen]

I can definitely do that if you want. The resulting image is indeed much smaller (so small it can run on a t3.nano along the nightscout instance ^^)

Want me to adapt the PR accordingly?

[timoschlueter]

@Gfeuillen: go ahead an make your image the new default. i think this would be a great improvement :)

[timoschlueter]

Closing this as stale.
The image size has been reduced with PR #136.