Add Docker build context to Diff check

[guineveresaenger]

Hello!

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Issue details

When the Docker build context (AKA the project folder referenced by the Dockerfile) has changed, an image should be re-built.

We can put this logic in the Diff checker.

Here's an example implementation for detecting changes in the Docker context: https://github.com/MaterializeInc/pulumi-docker-buildkit/blob/bed3b04f79016001c0b948d0faa1051857acd6fc/cmd/pulumi-resource-docker-buildkit/main.go#L127

• it heavily relies on keeping your node modules in the Dockerignore file, so this might be helpful to point out to users

Affected area/feature