Fix unbound variable when running container_image targets

[UebelAndre]

PR Checklist

Please check if your PR fulfills the following requirements:

• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Documentation content changes
• Other... Please describe:

What is the current behavior?

Currently running something like the following produces an error container_args[@]: unbound variable

bazel run //tests/container/python3:py3_image -- --rm -it --entrypoint /bin/bash

Issue Number: This is a minor bugfix to #1957

What is the new behavior?

This fixes the issue and I'm able to run my container and start a bash shell based on the arguments above.

Does this PR introduce a breaking change?

• Yes
• No

Other information