ci: use whole word matching for TARGET_FILTER

[moetayuko]

If the target name is a substring of another (e.g. malta/be and malta/be64), pushing the current target image will mistakenly override the other one.

Fixes #142

example broken action: https://github.com/openwrt/docker/actions/runs/10017123606

[moetayuko]

@aparcar can you handle this?

[aparcar]

This doesn't seem to work if the regex is empty. Could you change the default here?

[moetayuko]

what about the updated impl?

[aparcar]

Thanks let's see how it goes