Hardcode the OPAMROOT

It shouldn't be necessary since it's the default, but might help with child
images that redefine HOME (apparently Github actions does that).

Dockerfile

@@ -26,6 +26,7 @@ USER ocaml
 WORKDIR /home/ocaml
 ENV OPAMYES 1
 ENV OPAMDEPEXTYES 1
+ENV OPAMROOT /home/ocaml/.opam
 
 ARG LAST_REPO_UPDATE=0
 RUN echo "LAST_REPO_UPDATE=${LAST_REPO_UPDATE}"