implement first pass at cached image data source (#3)

implements envbuilder_cached_image data source

GNUmakefile

@@ -17,3 +17,34 @@ build: terraform-provider-envbuilder
 
 terraform-provider-envbuilder: internal/provider/*.go main.go
 	CGO_ENABLED=0 go build .
+
+.PHONY: update-envbuilder-version
+update-envbuilder-version:
+	go get github.com/coder/envbuilder@main
+	go mod tidy
+
+# Starts a local Docker registry on port 5000 with a local disk cache.
+.PHONY: test-registry
+test-registry: test-registry-container test-images-pull test-images-push
+
+.PHONY: test-registry-container
+test-registry-container: .registry-cache
+	if ! curl -fsSL http://localhost:5000/v2/_catalog > /dev/null 2>&1; then \
+		docker rm -f tfprov-envbuilder-registry && \
+		docker run -d -p 5000:5000 --name envbuilder-registry --volume $(PWD)/.registry-cache:/var/lib/registry registry:2; \
+	fi
+
+# Pulls images referenced in integration tests and pushes them to the local cache.
+.PHONY: test-images-push
+test-images-push: .registry-cache/docker/registry/v2/repositories/test-ubuntu
+
+.PHONY: test-images-pull
+test-images-pull:
+	docker pull ubuntu:latest
+	docker tag ubuntu:latest localhost:5000/test-ubuntu:latest
+
+.registry-cache:
+	mkdir -p .registry-cache && chmod -R ag+w .registry-cache
+
+.registry-cache/docker/registry/v2/repositories/test-ubuntu:
+	docker push localhost:5000/test-ubuntu:latest