nix: experimental support for building a Docker image

Run using something like:

```
docker run \
  --device nvidia.com/gpu=all \
  -it --rm -p 8080:80 \
  -v $PWD/data:/data \
  -v $PWD/tmp:/tmp \
  tgi-docker:latest \
  --model-id <model_id>
```

flake.nix

@@ -141,7 +141,8 @@
           };
         };
 
-        packages.default = pkgs.writeShellApplication {
+        packages = rec {
+          default = pkgs.writeShellApplication {
             name = "text-generation-inference";
             runtimeInputs = [
               server
@@ -151,6 +152,21 @@
               ${launcher}/bin/text-generation-launcher "$@"
             '';
           };
+
+          dockerImage = pkgs.dockerTools.buildLayeredImage {
+            name = "tgi-docker";
+            tag = "latest";
+            config = {
+              EntryPoint = [ "${default}/bin/text-generation-inference" ];
+              Env = [
+                "HF_HOME=/data"
+                "PORT=80"
+              ];
+
+            };
+            contents = [ pkgs.cacert ];
+          };
+        };
       }
     );
 }