[services] Clean up build_base_image.sh

Summary: Relevant Linear issue here. Replaced [ ] with [[ ]] and added some double quotes to prevent globbing and word splitting.

Test Plan: ShellCheck, close reading.

Reviewers: ashoat, atul

Reviewed By: ashoat

Subscribers: palys-swm, Adrian, karol-bisztyga

Differential Revision: https://phab.comm.dev/D4420