Sleeper CLI does not use bash when invoking scripts directly

[patchwork01]

Description

We recently switched the CMD statements in the Sleeper CLI Dockerfiles to use the JSON-based exec form instead of the shell form:

• For builder image in: Issue 2855 - Add JDK and sources to builder Docker image #2856
• For environment image in: Issue 2683 - Install Nix in cdk-environment EC2 #2818

Since then, when we run a script directly with a command like sleeper builder ./sleeper/scripts/build/buildForTest.sh, if the script has no shebang we get this error:

exec /sleeper-builder/sleeper/scripts/build/buildForTest.sh: exec format error

Steps to reproduce

1. Install Sleeper CLI
2. Check out Sleeper Git repository inside sleeper builder image
3. Run sleeper builder ./sleeper/scripts/build/buildForTest.sh
4. See error

Expected behaviour

All commands run directly with sleeper builder or sleeper environment should be run in a Bash shell.