Add beta images

[sfackler]

Closes #14

[sfackler]

@alexcrichton Is there a rustup channel for each specific beta release?

[alexcrichton]

Not currently, no

[sfackler]

Is that kind of thing handled by CI or manually? Would it be doable to have a channel per beta?

[alexcrichton]

Hm I'm not sure what you mean? You can install a specific beta using beta-yyyy-mm-dd but why would we have a channel per beta?

[sfackler]

I'm asking about supporting rustup update 1.23.0-beta.1 in addition to rustup update 1.23.0.

[alexcrichton]

Technically it should be feasible (along with everything else) but does the beta date not work?

[sfackler]

That should work I think - it's just a tiny bit less clear that the right beta's getting picked up.

[beatgammit]

Any updates on this? I'd like to use the beta builds if possible. :)

[sdroege]

This sounds very useful, it would allow running CI with beta too to basically give free pre-release testing in addition to running on nightly and stable :)

[sfackler]

Now that we make beta releases automatically whenever a commit lands on the branch, I think we'd probably need some automation here to support beta docker images.

[TimDiekmann]

I wrote something about automation in #14 . If desired, I can make a PR out of it that replaces this one.