Set correct bundler config in qa image

What does this MR do and why?

Describe in detail what your merge request does and why.

• Makes sure bundle config is not overwritten by local .bundle folder when building image on local machine.
• Remove --jobs=$(nproc) option because this setting is also persisted. This is not desirable because if original image is built on a powerful machine, it can spawn too many jobs on say local machine with less amount of cores. According to doc it is also already a default option The number of gems Bundler can install in parallel. Defaults to the number of available processors

Ideally .bundle folder should be present in .dockerignore (already ignored by git), but since qa image uses global .dockerignore, there might be some logic that relies on on copying config from outside for other builds.

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.