Fix docker image scanning action

Author: pvannierop

.github/workflows/scheduled-snyk-docker.yaml

@@ -20,7 +20,7 @@ jobs:
       - uses: actions/checkout@v5
 
       - name: Lowercase image name
-        run: echo "DOCKER_IMAGE=ghcr.io/${GITHUB_REPOSITORY,,}/management-portal" >> ${GITHUB_ENV}
+        run: echo "DOCKER_IMAGE=${DOCKER_IMAGE,,}" >> ${GITHUB_ENV}
 
       - name: Run Snyk to check for vulnerabilities
         continue-on-error: true # To make sure that SARIF upload gets called