Change the concrete helper image to use shell-form CMD

What does this MR do?

Fixes the concrete helper image by switching from shell-form CMD to exec-form CMD in the Dockerfile.

The concrete helper Dockerfile used shell-form CMD which Docker stores as ["/bin/sh", "-c", "..."], but the concrete base image is FROM scratch with no shell available. This MR switches to exec-form CMD and hardcodes the path since DST_DIR/DST_SUFFIX are only needed for Windows which will have its own concrete Dockerfile.

Why was this MR needed?

The Pilot Runner was failing with the following error when trying to use the concrete helper image:

ERROR: Job failed (system failure): prepare environment: Error response from daemon: 
failed to create task for container:
failed to create shim task: OCI runtime create failed: 
runc create failed: unable to start container process: error during container init: 
exec: "/bin/sh": stat /bin/sh: no such file or directory: unknown (exec.go:72:0s). 
Check https://docs.gitlab.com/runner/shells/#shell-profile-loading for more information

See failing job: https://gitlab.com/gitlab-org/step-runner/-/jobs/13728303136