Set OPAMDEPEXTYES, we want it in containers

Dockerfile

@@ -33,6 +33,7 @@ RUN echo 'ocaml ALL=(ALL) NOPASSWD: ALL' >> /etc/sudoers
 USER ocaml
 WORKDIR /home/ocaml
 ENV OPAMYES 1
+ENV OPAMDEPEXTYES 1
 
 ARG LAST_REPO_UPDATE=0
 RUN echo "LAST_REPO_UPDATE=${LAST_REPO_UPDATE}"

README.md

@@ -12,6 +12,9 @@ The images only contain:
 No switches are pre-created, but you are encouraged to use the provided system
 compiler to avoid re-building OCaml.
 
+The default user is named `ocaml`, has `sudo` rights, and opam's external
+dependency handling is enabled (`OPAMDEPEXTYES=true`).
+
 The images are re-generated every week, which should avoid the need to run `opam
 update`. You can typically start your project test scripts with: