Fix Debian GPG key issue: add debian-archive-keyring

- Install curl first before testing apt-cacher-ng connectivity
- Add debian-archive-keyring package to fix GPG signature verification
- This resolves the 'repository not signed' error from run #17
- Follows the same pattern as working deb-mock workflow

The Debian container was missing GPG keys needed to verify
repository signatures, causing apt-get update to fail.

.forgejo/workflows/ci.yml

@@ -41,6 +41,11 @@ jobs:
           bash -c '
         set -e
         
+        # Install curl first
+        echo "Installing curl..."
+        apt-get update
+        apt-get install -y curl
+        
         # Try apt-cacher-ng first, fallback to Debian's automatic mirror selection
         echo "Checking for apt-cacher-ng availability..."
         
@@ -57,6 +62,11 @@ jobs:
           echo "Using httpredir.debian.org for automatic mirror selection"
         fi
         
+        # Add Debian GPG keys
+        echo "Adding Debian GPG keys..."
+        apt-get update
+        apt-get install -y debian-archive-keyring
+        
         # Update and install remaining dependencies
         apt-get update
         apt-get install -y \