docker: add libgcc runtime dependency to prevent pthread_exit abort

[soroushalinia]

Fixes #23

When connecting via SSH (e.g. for tunneling), the container could crash with:
"libgcc_s.so.1 must be installed for pthread_exit to work"

Root cause: the final runtime image was missing libgcc_s.so.1.

Change:

• Copy libgcc_s.so.1 from the builder stage into the runtime stage:
  COPY --from=builder /lib/x86_64-linux-gnu/libgcc_s.so.1 /lib/x86_64-linux-gnu/

Tested:

• Built the Docker image locally and verified SSH connect/disconnect no longer terminates the container.

[EndPositive]

prefer using the cc distroless image, see dd11646