Add `cacheTo` argument to `ci` action

[sebst]

When building a devcontainer image using the ci action, I want to be able to generate a cache image, so that this can be used by other developers for faster devcontainer build times.

[torsknod2]

I tried to use the current main branch in my workflow and get the following error:

Run devcontainers/ci@3998f9ee7179e91ad72065e0b68fe2ebed581346
node:internal/modules/cjs/loader:1228
 throw err;
 ^

Error: Cannot find module './dist/index.js'
Require stack:
- /var/lib/private/gharsha/_actions/devcontainers/ci/3998f9ee7179e91ad720[65](https://github.com/torsknod2/MDSpanTest/actions/runs/12691429812/job/35374473201#step:12:66)e0b68fe2ebed581346/github-action/run-main.js
 at Module._resolveFilename (node:internal/modules/cjs/loader:1225:15)
 at Module._load (node:internal/modules/cjs/loader:1051:27)
 at Module.require (node:internal/modules/cjs/loader:1311:19)
 at require (node:internal/modules/helpers:179:18)
 at Object.<anonymous> (/var/lib/private/gharsha/_actions/devcontainers/ci/3998f9ee7179e91ad72065e0b[68](https://github.com/torsknod2/MDSpanTest/actions/runs/12691429812/job/35374473201#step:12:69)fe2ebed581346/github-action/run-main.js:1:21)
 at Module._compile (node:internal/modules/cjs/loader:14[69](https://github.com/torsknod2/MDSpanTest/actions/runs/12691429812/job/35374473201#step:12:70):14)
 at Module._extensions..js (node:internal/modules/cjs/loader:1548:10)
 at Module.load (node:internal/modules/cjs/loader:1288:32)
 at Module._load (node:internal/modules/cjs/loader:1104:12)
 at Function.executeUserEntryPoint [as runMain] (node:internal/modules/run_main:174:12) {
 code: 'MODULE_NOT_FOUND',
 requireStack: [
 '/var/lib/private/gharsha/_actions/devcontainers/ci/3998f9ee[71](https://github.com/torsknod2/MDSpanTest/actions/runs/12691429812/job/35374473201#step:12:72)79e91ad72065e0b68fe2ebed581346/github-action/run-main.js'
 ]
}

Node.js v20.18.0

I guess this is because the build tags contain the generated JavaScript files, while the commit does not.

However, looking at the last builds here, they also fail, even before this feature got introduced.
Is anyone using this somehow?
Would be great to have an example even before there is a release with the feature.

[sebst]

Just for reference on how to set up prebuilt images for dev containers with GitHub Actions:

devcontainer.community/20250303-prebuild-devcontainer/