Avoid error if /opt already exists

This fixes the build error:
https://gitlab.com/gitlab-org/gitlab-build-images/-/jobs/156906378

Dockerfile.gitlab-charts-build-base

@@ -10,7 +10,7 @@ ENV KUBECTL_URL=https://storage.googleapis.com/kubernetes-release/release/v${KUB
 
 # Install dependencies
 RUN apk --no-cache add -U openssl curl tar gzip bash ca-certificates git python2\
-  && mkdir /opt
+  && mkdir -p /opt
 
 # Install kubectl
 RUN curl -L -o /usr/local/bin/kubectl ${KUBECTL_URL} \