devcontainer build does not use docker build cache

[cwharris]

devcontainer build chooses to use a different --build-context on each build, forcing Docker to build from scratch each time, even if only a single feature was modified.

$ devcontainer build ~/dev/project/.devcontainer/ --workspace-folder ~/dev/project/
[10 ms] @devcontainers/cli 0.25.2. Node.js v17.0.1. linux 5.10.0-1050-oem x64.
[12164 ms] Start: Run: docker buildx build
    --load \
    --build-context dev_containers_feature_content_source=/tmp/devcontainercli-charris/container-features/0.25.2-1670206334608 \
    --build-arg _DEV_CONTAINERS_BASE_IMAGE=ubuntu:focal \
    --build-arg _DEV_CONTAINERS_IMAGE_USER=root \
    --build-arg _DEV_CONTAINERS_FEATURE_CONTENT_SOURCE=dev_container_feature_content_temp \
    --target dev_containers_target_stage
    -t vsc-project-de9db83a4d84fadc3ffbe532c095a7c7-features
    -f /tmp/devcontainercli-charris/container-features/0.25.2-1670206334608/Dockerfile.extended
    /tmp/devcontainercli-charris/empty-folder

I discovered this while working on a custom feature that depended on multiple time-consuming-to-build features, wasting several hours waiting on builds trying to figure out why no steps of the build were being cached.

This makes devcontainers impractical for me, since I need to develop quite a few features to support the work I'm doing. If each single character change to an install script causes an uncached build of the entire container, developing features becomes infeasible.

cli/src/spec-node/utils.ts

Line 432 in 0a3e2dc

const tmpFolder: string = cliHost.path.join(await getCacheFolder(cliHost), 'container-features', `${version}-${Date.now()}`);

What happens if we just remove Date().now from tmpFolder name?

[chrmarti]

Not sure the new folder plays a role here. We are using a single layer for building the features which is tracked in #291. Continuing there. Thanks.