Fix gitlab-runner mk-prebuilt-imags.sh

Original command 

```sh
cdebootstrap \
    --flavour=minimal \
    --exclude="dmsetup,e2fsprogs,init,systemd-sysv,systemd,udev" \
    --include="bash,ca-certificates,git,netcat-traditional" \
    stable ./debian-minbase

```

fail due to missing default mirror.

Fix by adding the mirror as the end of the command.

```sh
cdebootstrap \
    --flavour=minimal \
    --exclude="dmsetup,e2fsprogs,init,systemd-sysv,systemd,udev" \
    --include="bash,ca-certificates,git,netcat-traditional" \
    # Use debian mirror.
    stable ./debian-minbase  http://deb.debian.org/debian/
```