Update Go version to 1.26 and adjust workflows and Dockerfile accordingly

Signed-off-by: Marc Schäfer <git@marcschaeferger.de>

.github/workflows/test.yml

@@ -16,10 +16,14 @@ jobs:
     steps:
       - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
 
+      - name: Read go version
+        id: goversion
+        run: echo "version=$(cat .go-version)" >> $GITHUB_OUTPUT
+
       - name: Set up Go
         uses: actions/setup-go@4b73464bb391d4059bd26b0524d20df3927bd417 # v6.3.0
         with:
-          go-version: 1.25
+          go-version: ${{ steps.goversion.outputs.version }}
 
       - name: Build go
         run: go build