Prohibit image usage in local backend

[zc-devs]

Clear and concise description of the problem

Currently, image clause is used as a path to the executable.

Suggested solution

Use entrypoint or introduce new special clause for Local backend.

Alternative

Get rid of image and use only commands.

Additional context

#3263 (reply in thread) (point N3)

Validations

• Checked that the feature isn't part of the next version already [https://woodpecker-ci.org/faq#which-version-of-woodpecker-should-i-use]
• Read the docs.
• Check that there isn't already an issue that request the same feature to avoid creating a duplicate.

[qwerty287]

This needs a depreciation first, can't be removed in 3.0 directly.

Also, something to note: the schema currently says the image is required. We would need to drop that and that makes it less helpful cod linting.

Get rid of image and use only commands.

This should not be done because you can't use a command like export x=y then anymore.

[zc-devs]

This needs a depreciation first, can't be removed in 3.0 directly

Sad, but OK.

The main point is to get rid of confusing image. Do something like environment was removed from plugins and schema was corrected (#3909).
So, there is entrypoint for shell and commands for whatever else, then.