feat: Add RISC-V64 to Dockerfile template

[cpswan]

This will help towards #96, though there are a bunch of things that need to be resolved first, so making this a draft for now:

• debian:bullseye-slim (Stable) doesn't support RISC-V. There's support in debian:unstable-slim (Sid), but that's way off for now as there's not even a release date for the present testing build Bookworm.
• The scripts and tests need to be updated. I made a start on this, but it looks like everything assumes that architectures are present in stable and beta channels. I can't see a way to introduce a new architecture just in beta (for now).

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https://dart-review.googlesource.com/c/dart-docker/+/291161 has been updated with the latest commits from this pull request.

[cpswan]

Having tested build images based on this, I'm removing the draft status. It's ready to go when the other pieces slide into place.

Opened #141 to flag what might be a RISC-V specific issue found as a result of that testing.

[copybara-service[bot]]

https://dart-review.googlesource.com/c/dart-docker/+/291161 has been updated with the latest commits from this pull request.

[athomas]

Comment on Gerrit:

Hi Chris, thanks for sending this PR!

We discussed a bit, and we'd like to wait with merging this until RISC-V is supported in our stable channel. I hope that is OK.

[cpswan]

Of course that's OK @athomas

As I noted above there are a bunch of upstream dependencies that need to stabilise. There's a slim chance that will happen before RISC-V gets into stable channel, but my expectation is that RISC-V will be in Dart stable before it's in Debian stable.

[natebosch]

@athomas - is there any issue we can link to to track when this PR can move forward?

[devoncarew]

Closing as both the corresponding gerrit CLs have been closed. Please feel free to re-contribute (based on feedback from @athomas).