Update build pipeline.

docker/natter/Dockerfile

@@ -1,18 +1,24 @@
+# syntax=docker/dockerfile:1
 #
 # Builder
 #
+ARG NIX_SUBSTITUTERS=https://cache.nixos.org
 
 FROM nixos/nix:2.31.3 AS builder
 
-RUN cp "$(nix --extra-experimental-features "nix-command flakes" --option filter-syscalls false build nixpkgs#cacert --print-out-paths)/etc/ssl/certs/ca-bundle.crt" /tmp/ca-bundle.crt
+ARG NIX_SUBSTITUTERS
+RUN tee -a /etc/nix/nix.conf <<EOF
+extra-experimental-features = nix-command flakes
+filter-syscalls = false
+substituters = $NIX_SUBSTITUTERS
+EOF
+
+RUN cp "$(nix build nixpkgs#cacert --print-out-paths)/etc/ssl/certs/ca-bundle.crt" /tmp/ca-bundle.crt
 
 COPY . /tmp/build
 WORKDIR /tmp/build
 
-RUN nix \
-    --extra-experimental-features "nix-command flakes" \
-    --option filter-syscalls false \
-    build '.#docker_env'
+RUN --mount=type=ssh GIT_SSH_COMMAND="ssh -o StrictHostKeyChecking=no -o UserKnownHostsFile=/dev/null" nix build '.#docker_env'
 
 # Export the built closure to a folder
 RUN mkdir /tmp/nix-store-closure
@@ -36,5 +42,5 @@ COPY --from=builder /tmp/ca-bundle.crt /etc/ssl/certs/ca-bundle.crt
 
 COPY --from=builder /tmp/nix-store-closure /nix/store
 COPY --from=builder /tmp/build/result /app
-EXPOSE 8080
+
 CMD ["/app/bin/natter"]