[feat]: support build for ARM architectures

[oreoluwa]

image: oreoluwa/shadowbox

Just a reference implementation for building for the arm architecture.

[googlebot]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project (if not, look below for help). Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

📝 Please visit https://cla.developers.google.com/ to sign.

Once you've signed (or fixed any issues), please reply here with @googlebot I signed it! and we'll verify it.

---

What to do if you already signed the CLA

Individual signers

• It's possible we don't have your GitHub username or you're using a different email address on your commit. Check your existing CLA data and verify that your email is set on your git commits.

Corporate signers

• Your company has a Point of Contact who decides which employees are authorized to participate. Ask your POC to be added to the group of authorized contributors. If you don't know who your Point of Contact is, direct the Google project maintainer to go/cla#troubleshoot (Public version).
• The email used to register you as an authorized contributor must be the email used for the Git commit. Check your existing CLA data and verify that your email is set on your git commits.
• The email used to register you as an authorized contributor must also be attached to your GitHub account.

ℹ️ Googlers: Go here for more info.

[JonathanDCohen]

This file is almost identical to install_server.sh. Let's add a --arm flag? or --platform={arm,x86}

[JonathanDCohen]

The Prometheus args are only needed in the second build stage, let's move them down there?

[JonathanDCohen]

Any particular reason for this? Especially since we don't have ARM machines to test on, I'd lean towards removing this and the associated apk add call unless it's necessary

[oreoluwa]

@JonathanDCohen totally agree with all the comments above, thanks for reviewing.
PS:

1. this PR was done/raised before the first build of the outline-ss-server for the arm arch.
2. it wasn't really meant as a PR, but a reference implementation for anyone wanting to build the outline-server for their arm devices, based on my experience while trying to run outline on my RaspberryPi.
3. I can clean this up in the coming week, with the updated binaries (since prometheus and outline-ss-server now have official arm releases) and incorporate your comments above, if support for arm is currently being considered.

[JonathanDCohen]

@oreoluwa We're definitely down for best-effort ARM support although it isn't currently in our CI suite. I'll take a look whenever you get around to updating :)

[googlebot]

We found a Contributor License Agreement for you (the sender of this pull request), but were unable to find agreements for all the commit author(s) or Co-authors. If you authored these, maybe you used a different email address in the git commits than was used to sign the CLA (login here to double check)? If these were authored by someone else, then they will need to sign a CLA as well, and confirm that they're okay with these being contributed to Google.
In order to pass this check, please resolve this problem and then comment @googlebot I fixed it.. If the bot doesn't comment, it means it doesn't think anything has changed.

ℹ️ Googlers: Go here for more info.

[andela-oakinniranye]

@JonathanDCohen implemented the multi-arch build. However, I'm not sure I fully understand the CI setup yet, but I'd propose we do a multi-arch deployment, with docker manifest, that way the user wouldn't need to specify the tag for the image to be deployed and can just use the stable or daily* image; seems that's the approach watchtower and Prometheus also took: https://github.com/containrrr/watchtower/blob/master/.circleci/config.yml#L133-L206
https://github.com/prometheus/prometheus/blob/master/Makefile.common#L211-L233
PS: this PR also enables the option to specify the WATCHTOWER_IMAGE image, as the default v2tech/watchtower has been replaced with containrrr/watchtower which has a multi-arch support as well.