feat: Add validation for docker-cmd-file

[lazytesting]

related to #817

• Validates if docker-cmd-file exists, is readable and has execute permission.
• Made readme a bit more explicit regarding execute rights

[lazytesting]

i've updated a little, so you only need to do the async implementation

thanks for that, I added the async implementation

[viceice]

i've updated a little, so you only need to do the async implementation

thanks for that, I added the async implementation

please use the re-request review button on PR to get another review

[viceice]

Marked as draft due to inactivity.