feat(arm): Cross build arm and arm64 docker images

[karlskewes]

Description

• Add additional .arm64 and .armv6 Dockerfiles
• .gitignore release/ directory

Could be tidied by looping arch's in Makefile and creating a multi-arch manifest (requires docker cli experimental features enabled).

Motivation and Context

Second half of #16

How Has This Been Tested?

make release
make docker-all

$ docker inspect pusher/oauth2_proxy:latest{,-amd64,-arm64,-armv6} | grep Arch
        "Architecture": "amd64",
        "Architecture": "amd64",
        "Architecture": "arm64",
        "Architecture": "arm",

Run docker image on RPi 3B:

karl@pi1:~ $ docker run pusher/oauth2_proxy:latest-armv6 -version
oauth2_proxy v3.0.0-18-g2bdf00a-dirty (built with go1.11.5)
karl@pi1:~ $ docker run pusher/oauth2_proxy:latest-armv6 
2019/02/01 20:33:47 main.go:107: Invalid configuration:
  missing setting: cookie-secret
  missing setting: client-id
  missing setting: client-secret
  missing setting for email validation: email-domain or authenticated-emails-file required.
      use email-domain=* to authorize all email addresses

Run docker image on Rock64:

rock64@k8s-w-01:~$ sudo docker run pusher/oauth2_proxy:latest-arm64 -version
oauth2_proxy v3.0.0-18-g2bdf00a-dirty (built with go1.11.5)
rock64@k8s-w-01:~$ sudo docker run pusher/oauth2_proxy:latest-arm64 
2019/02/01 20:34:44 main.go:107: Invalid configuration:
  missing setting: cookie-secret
  missing setting: client-id
  missing setting: client-secret
  missing setting for email validation: email-domain or authenticated-emails-file required.
      use email-domain=* to authorize all email addresses

Checklist:

• My change requires a change to the documentation or CHANGELOG.
• I have updated the documentation/CHANGELOG accordingly.
• I have created a feature (non-master) branch for my PR.

[JoelSpeed]

TIL you can run arm containers on docker for mac and it doesn't complain at you.

I have no way of testing these until I get back to the office on monday but from my understanding they seem sane and the containers I've built claim to be arm, so if you're confident that they work I'm happy to merge

[karlskewes]

Wow, I haven't seen that before. But I do get the same behaviour running arm64 binary on amd64.

Doesn't work the other way though, amd64 binary on arm:

karl@pi1:~ $ ./oauth2_proxy-linux-amd64 
-bash: ./oauth2_proxy-linux-amd64: cannot execute binary file: Exec format error

rock64@k8s-w-01:~$ ./oauth2_proxy-linux-amd64 
-bash: ./oauth2_proxy-linux-amd64: cannot execute binary file: Exec format error

Otherwise yeah I think this is good to go. No rush if you want to wait until back in office.

[JoelSpeed]

Just tested this on a Pi V3 and it looks like it's working to me, so happy for this to get merged!

Thanks @kskewes