fixed specifying image(s) to be build and pushed

[henninggross]

Fixes an issue that came up due to changes made in #287 which resulted in all images being built and pushed even when running

./scripts/build-and-oush-images.sh --image foo

Fixes #299

Tasks:

• Updated design documents in docs/design directory or not applicable
• Updated user-facing documentation in docs directory or not applicable
• Ran tests (e.g. make test) or not applicable
• Updated changelog or not applicable

[henninggross]

I am not that familiar with bash still, so feedback is highly appreciated on this one.

[michaelsauter]

Do you know https://www.shellcheck.net? This can help to uncover some "mistakes" :) See also #240.

The change looks good. The combination of loop / global variable / function invocation makes me a bit uneasy, but I also would have to look up how to make it better.

Let me know if you want to do a 2nd round with my recommendations above, otherwise this LGTM.

[henninggross]

Actually I did not know that tool, thanks for pointing me to it.

I am happy to apply the changes it proposes (missing double quotes) but when it comes to the structure of the code I currently don't have the knowledge on shell best practices and would leave it as is.
Glad for anyone to take this up if they see a need though.