fix: images/mkdocs/Dockerfile to reduce vulnerabilities

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/SNYK-ALPINE316-BUSYBOX-6913410
- https://snyk.io/vuln/SNYK-ALPINE316-BUSYBOX-6913410

images/mkdocs/Dockerfile

@@ -12,7 +12,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-FROM alpine:3.16
+FROM alpine:3
 
 RUN apk add --no-cache \
   bash \