fix: Dockerfile to reduce vulnerabilities

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/SNYK-ALPINE318-OPENSSL-8235200
- https://snyk.io/vuln/SNYK-ALPINE318-OPENSSL-8235200

.github/workflows/build.yml

@@ -66,7 +66,7 @@ jobs:
 
       - name: Build and push
         id: build
-        uses: docker/build-push-action@v6
+        uses: docker/build-push-action@v5
         with:
           builder: ${{ steps.buildx.outputs.name }}
           context: .

Dockerfile

@@ -47,7 +47,7 @@ RUN go install github.com/yaegashi/muslstack@latest && \
 
 # ---
 
-FROM alpine:3.18
+FROM alpine:3.21.0
 
 # renew global args from above & pin any dependency versions
 ARG HUGO_VERSION