feat(build): Support additional pre-launch commands in image

This makes it possible for users to hook basically arbitrary things
into the Nixery container image.

default.nix

@@ -11,7 +11,8 @@
 # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 # See the License for the specific language governing permissions and
 # limitations under the License.
-{ pkgs ? import <nixpkgs> {} }:
+{ pkgs ? import <nixpkgs> {}
+, preLaunch ? "" }:
 
 with pkgs;
 
@@ -88,6 +89,8 @@ rec {
       mkdir -p /etc/nix
       echo 'sandbox = false' >> /etc/nix/nix.conf
 
+      ${preLaunch}
+
       exec ${nixery-bin}/bin/nixery
     '';
   in dockerTools.buildLayeredImage {