Update python version used in dockerfile from python 3.7 to 3.8

[lewisosborne]

• simple update of the python version used in this repository's dockerfile from python 3.7 to 3.8
• the reason for this is to make the airflow-dag-action more compatible with dependency requirements of Airflow-related packages (for example, airflow-provider-fivetran-async) that have been more recently updated & require python version >= 3.8.
• this builds successfully locally via docker build .:

[jayamanikharyono]

Approved, Just get back to this project and checking for updates, please let me know if you have some request, Thank you for this contribution

[lewisosborne]

Approved, Just get back to this project and checking for updates, please let me know if you have some request, Thank you for this contribution

Many thanks 🫡

[lewisosborne]

Approved, Just get back to this project and checking for updates, please let me know if you have some request, Thank you for this contribution

@jayamanikharyono I do have one request - could you please create a new release version of this repo that includes this latest PR merge? (i.e. v2.5). I will then be able to reference it properly - the current release, v2.4, does not include the latest changes

Many thanks!