Dockerfile.aarch64 for building for Docker on ARMv8

[vielmetti]

Description

Proposed to create a Dockerfile.aarch64 for building on ARMv8, akin to the existing Raspberry Pi image.

The current Dockerfile is built from Alpine 3.4. The current version of Alpine is 3.5.2. The supportable Alpine images on https://hub.docker.com/r/aarch64/alpine/tags/ include a 3.5 tag, but not a 3.4 tag, so there might reasonably be other issues involved in the move from 3.4 to 3.5.

Other than that I don't anticipate any big issues....

[tboerger]

Already got the plan, like Gogs at gogs/gogs#4018

[tboerger]

And I think we will update all containers to alpine 3.5

[vielmetti]

Aha, looks like https://hub.docker.com/r/atzoum/aarch64-gitea/ is a working example from @atzoum

[lunny]

This may duplicated with #691 ?

[tboerger]

Not related to #691, this issue only wants a working Dockerfile

[lunny]

resolved by #1130