Add experimental armhf support via "case" on "apk --print-arch"

[tianon]

This allows for the same source to build on both amd64 and armhf -- I'm not 100% sure whether Docker's static binary releases are ARMv6 or ARMv7, and I've only got an ARMv7 environment to test in. 😇

Here's some docker version and docker info output from a running instance of this DinD on an ARM machine:

$ docker -H tcp://172.17.0.2:2375 version
Client:
 Version:      1.12.6
 API version:  1.24
 Go version:   go1.6.2
 Git commit:   78d1802
 Built:        Tue Jan 31 23:35:09 2017
 OS/Arch:      linux/arm

Server:
 Version:      17.03.1-ce
 API version:  1.27
 Go version:   go1.7.5
 Git commit:   c6d412e
 Built:        Tue Mar 28 03:25:54 2017
 OS/Arch:      linux/arm
$ docker -H tcp://172.17.0.2:2375 info
Containers: 0
 Running: 0
 Paused: 0
 Stopped: 0
Images: 0
Server Version: 17.03.1-ce
Storage Driver: aufs
 Root Dir: /var/lib/docker/aufs
 Backing Filesystem: extfs
 Dirs: 0
 Dirperm1 Supported: true
Logging Driver: json-file
Cgroup Driver: cgroupfs
Plugins:
 Volume: local
 Network: bridge host macvlan null overlay
Swarm: inactive
Runtimes: runc
Default Runtime: runc
Security Options: apparmor seccomp
Kernel Version: 4.4.0-1040-raspi2
Operating System: Alpine Linux v3.5 (containerized)
OSType: linux
Architecture: armv7l
CPUs: 4
Total Memory: 729.7 MiB
Name: be2cc3b1a5d0
ID: GCV6:KPUT:HA7C:GRHD:63ZJ:L6KR:F6YW:24OQ:I2ME:W6MY:RRXT:WGLY
Docker Root Dir: /var/lib/docker
Debug Mode (client): false
Debug Mode (server): false
Registry: https://index.docker.io/v1/
WARNING: No cpuset support
WARNING: bridge-nf-call-iptables is disabled
WARNING: bridge-nf-call-ip6tables is disabled
Insecure Registries:
 127.0.0.0/8

[yosifkit]

Merging since no functional change to x86 images, but gives access to build arm images. 🎉

[jcberthon]

That's very cool, so I do not have to write now for each new Docker version a dedicated ARM Dockerfile.

However, I have the following question still: are they any plan to also push the ARM image to Docker hub?

[tianon]

@jcberthon unfortunately, no -- see #54 (switching from get.docker.com to download.docker.com)

These armhf binaries were supported by the Docker Project, but not Docker Inc, so do not exist on download.docker.com.