Set the shiny new OPAMCONFIRMLEVEL var

Dockerfile

 FROM alpine:3.11 as compilation
 
-ARG OCAML_VERSION=4.11.1
+ARG OCAML_VERSION=4.12.0
 
 RUN apk add gcc g++ make coreutils openssl
 ADD https://github.com/ocaml/ocaml/archive/${OCAML_VERSION}.tar.gz /src/ocaml.tar.gz
@@ -27,6 +27,7 @@ USER ocaml
 WORKDIR /home/ocaml
 ENV OPAMYES 1
 ENV OPAMDEPEXTYES 1
+ENV OPAMCONFIRMLEVEL unsafe-yes
 ENV OPAMROOT /home/ocaml/.opam
 
 ARG LAST_REPO_UPDATE=0