Update Dockerfile

[isarrider]

Define target build dir as tmpfs (makes build a little faster)
Be more specific by using FROM alpine:latest instead only FROM alpine

[SchumacherFM]

Can this be please merged to speed up docker build @andig ?

[andig]

Does this have any measurable effect? Does layer csching still work?

[SchumacherFM]

$ time docker buildx build --platform linux/arm64 -t ghcr.io/xxxxxxxxxx/mbmd:latest .
This PR

real	11m48.745s
user	0m2.571s
sys	0m2.225s

Without these changes:

real	10m55.284s
user	0m2.364s
sys	0m1.871s

OMG 🤣 - IMHO you can close this PR without merging. Sorry for bothering you.

[isarrider]

I cant remember the time difference but did you bench on sth with a very slow storage like a SD-Card?

[SchumacherFM]

MacBook Pro 13-inch, 2017 with Docker Desktop

[isarrider]

Well - when there is a fast SSD in -> of course no advantage
(besides not abusing the SSD Write cycles)
I use it on my Rock 5 with a slow SD Card...