Run build tests without the need of a test directory and Dockerfile

[elchenberg]

The following three lines of shell script could be replaced with a one-liner:

echo "FROM docker.io/library/hello-world:latest" | docker build --file=- .

dind-nurse/pkg/setup.go

Lines 88 to 90 in 9bde468

	`mkdir -p /test-build
	echo "FROM hello-world" > /test-build/Dockerfile
	docker build /test-build`,

echo "FROM docker.io/library/hello-world:latest" | docker buildx build --file=- .

dind-nurse/pkg/setup.go

Lines 109 to 111 in 9bde468

	`mkdir -p /test-build
	echo "FROM hello-world" > /test-build/Dockerfile
	docker buildx build /test-build`,