feat: Remove v prefix in Dockerfile _VERSION variables (#54)

Put an `x` into the box if that applies:

- [ ] This PR introduces breaking change.
- [ ] This PR fixes a bug.
- [ ] This PR adds new functionality.
- [x] This PR enhances existing functionality.

### How was it tested

It works the same as for GHA

default.template.json5

@@ -68,10 +68,9 @@
     "enabled": true
   },
   "packageRules": [
-    // Maybe useful. Get it from https://github.com/renovatebot/.github/blob/d9b3c1914f4bf9dbecc6456610ca89530260572f/default.json#L39-L44
     {
-      "description": "v prefix workaround for action updates",
-      "matchDepTypes": ["action"],
+      "description": "v prefix workaround for action and Dockerfile updates",
+      "matchDepTypes": ["action", "docker"],
       "extractVersion": "^(?<version>v\\d+\\.\\d+\\.\\d+)$",
       "versioning": "regex:^v(?<major>\\d+)(\\.(?<minor>\\d+)\\.(?<patch>\\d+))?$"
     },