Fix container image build issues for official releases

[ptitzler]

Closes #2841

What changes were proposed in this pull request?

• Perform a conditional copy of the wheel file instead of an unconditional one, which caused the container image builds to fail for official releases

How was this pull request tested?

Create container images from source code:

• git clone git@github.com:ptitzler/elyra.git
• cd elyra
• git checkout fix-docker-build
• Run make install (this is a current limitation unrelated to the issue this PR is addressing)
• Run make container-images (success)

Create container images from published packages:

• git clone git@github.com:elyra-ai/elyra.git into a different directory
• cd elyra
• git checkout tags/v3.10.1. # switch to a release tag (which does not yet contain the changes from this PR)
• copy etc/docker/kubeflow/Dockerfile (from this PR) to the same location in ^^
• copy etc/docker/kubeflow/Dockerfile (from this PR) to the same location in ^^
• Run make container-images (success)

Developer's Certificate of Origin 1.1

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[elyra-bot]

Thanks for making a pull request to Elyra!

To try out this branch on binder, follow this link:

[kevin-bates]

Thanks for solidifying this @ptitzler!

[kiersten-stokes]

Working like a charm!