[BUG] ImageSpec.is_container returns True during fast registration

### Flyte & Flytekit version

flytekit==1.16.1

### Describe the bug

The `ImageSpec` method `is_container()` returns True during fast registration with `pyflyte run --remote` or `pyflyte register`.

### Expected behavior

The `ImageSpec` method `is_container()` should return True only during execution in the corresponding container or during local execution.

### Additional context to reproduce

1. Implement the following workflow
```
from flytekit import ImageSpec, workflow, task

image_spec = ImageSpec(
packages=["pandas"],
registry=...
)

if image_spec.is_container():
import pandas as pd

@task(container_image=image_spec)
def task1():
df = pd.DataFrame()

@workflow
def wf1():
task1()
```

2. Create and activate a virtual environment with only `flytekit==1.16.1`

3. `pyflyte run --remote` or `pyflyte register` will try to execute the pandas import locally.

### Screenshots

_No response_

### Are you sure this issue hasn't been raised already?

- [x] Yes

### Have you read the Code of Conduct?

- [x] Yes