fix: empty image check (#543)

cibuildwheel/docker_container.py

@@ -30,6 +30,9 @@ class DockerContainer:
     bash_stdout: IO[bytes]
 
     def __init__(self, docker_image: str, simulate_32_bit: bool = False, cwd: Optional[PathOrStr] = None):
+        if not docker_image:
+            raise ValueError("Must have a non-empty docker image to run.")
+
         self.docker_image = docker_image
         self.simulate_32_bit = simulate_32_bit
         self.cwd = cwd