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 versionClient: 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/armServer: 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 infoContainers: 0 Running: 0 Paused: 0 Stopped: 0Images: 0Server Version: 17.03.1-ceStorage Driver: aufs Root Dir: /var/lib/docker/aufs Backing Filesystem: extfs Dirs: 0 Dirperm1 Supported: trueLogging Driver: json-fileCgroup Driver: cgroupfsPlugins: Volume: local Network: bridge host macvlan null overlaySwarm: inactiveRuntimes: runcDefault Runtime: runcSecurity Options: apparmor seccompKernel Version: 4.4.0-1040-raspi2Operating System: Alpine Linux v3.5 (containerized)OSType: linuxArchitecture: armv7lCPUs: 4Total Memory: 729.7 MiBName: be2cc3b1a5d0ID: GCV6:KPUT:HA7C:GRHD:63ZJ:L6KR:F6YW:24OQ:I2ME:W6MY:RRXT:WGLYDocker Root Dir: /var/lib/dockerDebug Mode (client): falseDebug Mode (server): falseRegistry: https://index.docker.io/v1/WARNING: No cpuset supportWARNING: bridge-nf-call-iptables is disabledWARNING: bridge-nf-call-ip6tables is disabledInsecure 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.