Fix container build workflows to checkout dispatch ref

When manually dispatching a container build with --ref, the build job
now checks out the specified commit instead of the branch HEAD. This
allows building containers from feature branches before merging.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.forgejo/workflows/build-container-nix.yaml

@@ -53,6 +53,8 @@ jobs:
     steps:
       - name: Checkout
         uses: actions/checkout@34e114876b0b11c390a56381ad16ebd13914f8d5 # v4.3.1
+        with:
+          ref: ${{ inputs.ref || github.sha }}
 
       - name: Check for default.nix
         id: check

.forgejo/workflows/build-container.yaml

@@ -54,6 +54,8 @@ jobs:
     steps:
       - name: Checkout
         uses: actions/checkout@34e114876b0b11c390a56381ad16ebd13914f8d5 # v4.3.1
+        with:
+          ref: ${{ inputs.ref || github.sha }}
 
       - name: Check for Dockerfile
         id: check