Change version / tag format to follow conventions

In order to allow for automated tooling to identify new releases of the
container image, follow best practices by adhering to the semver
specification (at least in terms of syntax).

Solves: #487

.github/workflows/main.yml

@@ -12,7 +12,7 @@ jobs:
       -
         name: Get current date
         id: date
-        run: echo "::set-output name=date::$(date +'%Y-%m-%d')"
+        run: echo "::set-output name=date::$(date +'%Y.%m.%d')"
       -
         name: Checkout
         uses: actions/checkout@v4