Mojo/solution_1 Dockerfile: Clean up after apt-get install

[rbergen]

Description

We overlooked that the apt-get install in the Dockerfile for Mojo/solution_1 was not followed by the appropriate clean-up. This fixes that.

Contributing requirements

• I read the contribution guidelines in CONTRIBUTING.md.
• I placed my solution in the correct solution folder.
• I added a README.md with the right badge(s).
• I added a Dockerfile that builds and runs my solution.
• I selected drag-race as the target branch.
• All code herein is licensed compatible with BSD-3.

[rbergen]

Merging based on out-of-band approval.