Improve dev Docker image

[peaceiris]

Is your feature request related to a problem? Please describe.

• https://github.com/peaceiris/actions-hugo/blob/main/.github/workflows/dev-image.yml
• https://github.com/peaceiris/actions-hugo/blob/main/Dockerfile
• related to #403
• Fixes for #471 https://github.com/peaceiris/actions-hugo/runs/1434423272#step:5:39

Describe the solution you'd like

Publish the dev Docker image.

Describe alternatives you've considered

N/A

Additional context

• cf. https://github.com/peaceiris/actions-gh-pages/blob/main/.github/workflows/dev-image.yml
• this may be useful: https://docs.github.com/en/free-pro-team@latest/actions/reference/workflow-syntax-for-github-actions#jobsjob_idcontainercredentials