Incrementally copy features near the layer they're installed

[trxcllnt]

This PR incrementally copies each feature's contents near the layer they're installed, so rebuilds of a devcontainer take full advantage of the build cache.

To illustrate the behavior I built a devcontainer, edited the source of the last feature in the image, then rebuilt it again. With the public devcontainer CLI, the image rebuilds all features even though only the last one changed. With this PR, the second build used the layer cache and started from the layer of the changed feature.

• build-1-old.log (1m31.803s)
• build-2-old.log (1m29.621s)
• build-1-new.log (1m27.219s)
• build-2-new.log (0m32.794s 🔥 🚀 🔥 🚀 🔥 🚀)

Fixes #291 and #313

[trxcllnt]

cc: @chrmarti

[jkeech]

@trxcllnt thanks for submitting this PR! This looks like a great improvement. I'd like @chrmarti to review, but just wanted to drop a comment saying it's on our radar and we should be getting to this soon. Thanks for your patience! 🙏

[theonlyfoxy]

It seems after changing dockerfile, Features installation is not using cache anymore. does this PR also fix that ?

[joshspicer]

Very awesome! Just validated this continues to work as expected when rebased against main.

Validated that changing the contents of the final Feature (in this case a local Feature) preserves all previously cached layers, and the change is picked up 👍