docker plugin: create completions directory if it doesn't exist

[mtowara]

Standards checklist:

• The PR title is descriptive.
• The PR doesn't replicate another PR which is already open.
• I have read the contribution guide and followed all the instructions.
• The code follows the code style guide detailed in the wiki.
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• The code is efficient, to the best of my ability, and does not waste computer resources.
• The code is stable and I have tested it myself, to the best of my abilities.
• If the code introduces new aliases, I provide a valid use case for all plugin users down below.

Changes:

• Fix Docker plugin fails to load with antigen #13679: Ensure the completions directory exists so the below cp / tee statements don't fail.

Other comments:

Instead of the check could also call mkdir -p every time. I've seen both behaviors in other plugins.
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