feat(cache): add initial nix cache shell program for using attic with actions

apps/ci-os/packages/cache/default.nix

@@ -0,0 +1,64 @@
+{ pkgs, lib, ... }:
+
+with pkgs;
+
+writeShellApplication rec {
+  name = "attic-action";
+
+  runtimeInputs = [ attic-client ];
+
+  # Define the shell script
+  text = ''
+    #!/usr/bin/env bash
+    set -euo pipefail
+
+    # Default values
+    ATTIC_ENDPOINT="${ATTIC_ENDPOINT:-https://cache.252.no}"
+    ATTIC_CACHE="${ATTIC_CACHE:-nix-gitops}"
+    ATTIC_TOKEN="${ATTIC_TOKEN:-}"
+    SKIP_PUSH="${SKIP_PUSH:-false}"
+    SKIP_USE="${SKIP_USE:-false}"
+
+    # Ensure required inputs are provided
+    if [[ -z "$ATTIC_ENDPOINT" || -z "$ATTIC_CACHE" ]]; then
+      echo "Error: Both ATTIC_ENDPOINT and ATTIC_CACHE are required."
+      exit 1
+    fi
+
+    if [[ -z "$ATTIC_TOKEN" ]]; then
+      echo "Warning: ATTIC_TOKEN is not set. Operations requiring authentication might fail."
+    fi
+
+    # Add the cache as a substituter unless SKIP_USE is true
+    if [[ "$SKIP_USE" != "true" ]]; then
+      echo "Adding $ATTIC_ENDPOINT as a substituter..."
+      attic use --endpoint "$ATTIC_ENDPOINT" "$ATTIC_CACHE" || {
+        echo "Error: Failed to add substituter."
+        exit 1
+      }
+    else
+      echo "Skipping use of cache as substituter."
+    fi
+
+    # Push to the cache unless SKIP_PUSH is true
+    if [[ "$SKIP_PUSH" != "true" ]]; then
+      echo "Pushing results to $ATTIC_ENDPOINT/$ATTIC_CACHE..."
+      attic push --endpoint "$ATTIC_ENDPOINT" "$ATTIC_CACHE" || {
+        echo "Error: Failed to push to cache."
+        exit 1
+      }
+    else
+      echo "Skipping cache push."
+    fi
+
+    echo "Attic action completed successfully."
+  '';
+
+  # Metadata for the package
+  meta = with lib; {
+    homepage = "https://code.252.no/tommy/containers";
+    description = "Binary cache server action script for CI pipelines";
+    license = licenses.mit;
+    maintainers = with maintainers; [ "tommy-skaug" ];
+  };
+}