fix: export the container default shell

[BrettKinny]

What changed

• Export SHELL=/bin/bash in the image, matching the default command.
• Add CI coverage that asserts the variable survives container startup.

Why

Some container-launched tools choose their child shell from SHELL. Docker does not guarantee that variable is exported when using docker exec, so the image could fall back to /bin/sh even though Squarebox's default command and shell configuration are Bash-based. That bypasses .bashrc and Starship initialization.

Validation

• docker build -t squarebox:shell-env-test .
• docker run --rm squarebox:shell-env-test env asserts SHELL=/bin/bash.
• git diff --check