freeze KUBECONTEXT for k8s_custom_deploy

[nicks]

Describe the Feature You Want

Right now, when tilt shells out in k8s_custom_deploy, tilt doesn't do anything to ensure that the script you're invoking is talking to the same kubernetes context that tilt is talking to.

This means that if you change the default context outside of tilt, you could accidentally deploy to a prod cluster 😱

Implementation Notes

We should change the cluster reconciler so that when it "resolves" the context, it:

1. Writes a new "frozen" kubeconfig, similar to kubectl config view --minify to a tempdir
2. Publish the path to the frozen kubeconfig on the cluster status field
3. When running k8s_custom_deploy, inject the KUBECONTEXT env variable to point to this new file.

[nicks]

i'll take a look at this since it came up again today.

[landism]

This problem could arise with local and local_resource as well, though that might be a bit dicier (e.g., I've written a Tiltfile that used local to run kubectl to copy secrets from one context to another, which could be annoying if Tilt forced the k8s config for all locals)

[landism]

This problem could arise with local and local_resource as well
A good argument here is that we've had multiple reports of it with k8s_custom_deploy in the short time since it shipped and haven't heard of it with local/local_resource in the years they've been around.

[kalmant]

This problem could arise with local and local_resource as well
A good argument here is that we've had multiple reports of it with k8s_custom_deploy in the short time since it shipped and haven't heard of it with local/local_resource in the years they've been around.

Someone using Tilt v0.27.0 just ran into this issue through helm_resource which made me realize local and local_resource are probably also at risk (and also led me to this issue).
So if possible, I'd like to cast a vote for a "fix" for local/local_resource. I'm sure I can put together some solution for us but a solution in tilt itself would be much better.

An idea: perhaps having a kubectl command in the API would work. It could make sure to always use the context you started Tilt with.