docker: explicit error on missing dockerfile

When the provided Dockerfile path was not present, users would get an
error when accessing the file, and if undefined, this would amount to
ENOENT on an empty path, which isn't too friendly.

Therefore, this commit treats this lack as a separate error, since
`path` is required when the bootstrap config is non-default, this will
provide more meaningful context on the failure and its remediation.

builder/docker/dockerfile_config.go

@@ -60,6 +60,10 @@ func (c *DockerfileBootstrapConfig) Prepare() ([]string, error) {
 		return nil, nil
 	}
 
+	if c.DockerfilePath == "" {
+		return nil, fmt.Errorf("`path` is required for bootstrapping a build with `docker build`")
+	}
+
 	if c.BuildDir == "" {
 		c.BuildDir = "."
 	}