Better handling of fallback not existing (#19)

* Don't check image tag if tag_fallback isn't specified

* Better handling of fallback not existing

action.yml

@@ -92,16 +92,18 @@ runs:
           echo "  b) triggers not provided"
           echo "triggered=true" >> $GITHUB_OUTPUT
           exit 0
-        fi
 
-        # Build if tag_fallback is no good (requires a valid container)
-        if [ $(docker buildx imagetools inspect ghcr.io/${{ inputs.repository }}/${{ inputs.package }}:${{ inputs.tag_fallback }} > /dev/null) -ne 0 ]
-        then
-            # Output triggered=true for next steps
-            echo "Build triggered.  Fallback tag (tag_fallback) not usable."
-            echo "Manifest checked for: ghcr.io/${{ inputs.repository }}/${{ inputs.package }}:${{ inputs.tag_fallback }}"
-            echo "triggered=true" >> $GITHUB_OUTPUT
-            exit 0
+        else
+          # Build if tag_fallback is no good (requires a valid container)
+          docker buildx imagetools inspect ghcr.io/${{ inputs.repository }}/${{ inputs.package }}:${{ inputs.tag_fallback }} > /dev/null 2>&1
+          if [ $? -ne 0 ]
+          then
+              # Output triggered=true for next steps
+              echo "Build triggered.  Fallback tag (tag_fallback) not usable."
+              echo "Manifest checked for: ghcr.io/${{ inputs.repository }}/${{ inputs.package }}:${{ inputs.tag_fallback }}"
+              echo "triggered=true" >> $GITHUB_OUTPUT
+              exit 0
+          fi
         fi
 
         # Build if changed files (git diff) match triggers