fix: only regard the positional arg as build ctx if it is a local dir (#5322)

Signed-off-by: Frost Ming <me@frostming.com>

Author: frostming

src/bentoml_cli/bentos.py

@@ -440,7 +440,7 @@ def build(  # type: ignore (not accessed)
             labels_dict[key] = label_value
 
         service: str | None = None
-        if ":" in build_ctx:
+        if ":" in build_ctx or not os.path.isdir(build_ctx):
             service = build_ctx
             build_ctx = "."