Add tests on docker.#Build definition

- Simple build test
- Multi step to build a go binary

Signed-off-by: Vasek - Tom C <tom.chauveau@epitech.eu>

pkg/universe.dagger.io/docker/test/testdata/main.go

@@ -0,0 +1,7 @@
+package main
+
+import "fmt"
+
+func main() {
+	fmt.Printf("hello world")
+}