docs: improve documentation for overwriting existing images (#5030)

Signed-off-by: jason.lai <[email protected]>

docs/user_guide/customizing_dependencies/imagespec.md

@@ -160,3 +160,17 @@ If you only want to build the image without registering the workflow, you can us
 ```
 pyflyte build --remote image_spec.py wf
 ```
+
++++
+
+In some cases, you may want to force an image to rebuild, even if the image spec hasn’t changed. If you want to overwrite an existing image, you can pass the `FLYTE_FORCE_PUSH_IMAGE_SPEC=True` to `pyflyte` command or add `force_push()` to the ImageSpec.
+
+```bash
+FLYTE_FORCE_PUSH_IMAGE_SPEC=True pyflyte run --remote image_spec.py wf
+```
+
+or
+
+```python
+image = ImageSpec(registry="ghcr.io/flyteorg", packages=["pandas"]).force_push()
+```