Running pre-built docker action does not provide required default values

Describe the bug
Running an action without providing required inputs that have a default will use the default values. Running the same action pre-built as a docker image does not show this behaviour.

To Reproduce
Steps to reproduce the behavior:

1. Use an action that has required inputs, e.g. EnricoMi/download-buildkite-artifact-action:

      uses: EnricoMi/download-buildkite-artifact-action@v1
      with:
        buildkite_token: ${{ secrets.BUILDKITE_TOKEN }}

2. The github_token input is defined required in the action.yml but not given here.

3. This makes the runner execute the action with the default value: https://github.com/horovod/horovod/runs/1912170696#step:4:2

Run EnricoMi/download-buildkite-artifact-action@v1
  with:
    buildkite_token: ***
    github_token: ***
    log_level: INFO

4. When I now switch over to the identical but pre-built action, the default values for missing inputs are not provided (e.g. github_token or log_level):

      docker://ghcr.io/enricomi/download-buildkite-artifact-action:v1
      with:
        buildkite_token: ${{ secrets.BUILDKITE_TOKEN }}

https://github.com/horovod/horovod/runs/1912156006#step:4:2

Run docker://ghcr.io/enricomi/download-buildkite-artifact-action:v1
  with:
    buildkite_token: ***

Expected behavior

Running the action should behave identical, no matter if run as pre-build docker image (docker://ghcr.io/enricomi/download-buildkite-artifact-action:v1) or from sources (EnricoMi/download-buildkite-artifact-action@v1).

Runner Version and Platform

Current version running at GitHub. Above example runs on ubuntu-latest.

Job Log Output

https://github.com/horovod/horovod/runs/1912156006#step:4:2
https://github.com/horovod/horovod/runs/1912170696#step:4:2

Compare the two cited workflow runs and the referenced action.yml to confirm how missing defaulted inputs differ between source-based and docker actions. Start from the runner's action-input handling and trace the docker action entry point; done means both invocation forms provide the same required defaults, including github_token and log_level, with a regression test covering the behavior.