[build infra][ocaml] disable opam sandboxing for ubuntu bionic and focal (#2671)

* disable opam sandboxing for ubuntu focal

* update bionic as well
diff --git a/build/docker/ubuntu-bionic/Dockerfile b/build/docker/ubuntu-bionic/Dockerfile
index a673351..679f509 100644
--- a/build/docker/ubuntu-bionic/Dockerfile
+++ b/build/docker/ubuntu-bionic/Dockerfile
@@ -197,7 +197,8 @@
       `# OCaml dependencies` \
       ocaml \
       opam && \
-      opam init --yes && \
+      `# disable sandboxing see https://github.com/ocaml/opam/issues/4327` \
+      opam init --yes --disable-sandboxing && \
       opam install --yes oasis
 
 RUN apt-get install -y --no-install-recommends \
diff --git a/build/docker/ubuntu-focal/Dockerfile b/build/docker/ubuntu-focal/Dockerfile
index 3dffc58..a9a9ab9 100644
--- a/build/docker/ubuntu-focal/Dockerfile
+++ b/build/docker/ubuntu-focal/Dockerfile
@@ -197,7 +197,8 @@
       `# OCaml dependencies` \
       ocaml \
       opam && \
-      opam init --yes && \
+      `# disable sandboxing see https://github.com/ocaml/opam/issues/4327` \
+      opam init --yes --disable-sandboxing && \
       opam install --yes oasis
 
 RUN apt-get install -y --no-install-recommends \