remove --disable-sandboxing in bionic (#2683)

diff --git a/build/docker/ubuntu-bionic/Dockerfile b/build/docker/ubuntu-bionic/Dockerfile
index c554c09..b786ab5 100644
--- a/build/docker/ubuntu-bionic/Dockerfile
+++ b/build/docker/ubuntu-bionic/Dockerfile
@@ -197,8 +197,7 @@
       `# OCaml dependencies` \
       ocaml \
       opam && \
-      `# disable sandboxing see https://github.com/ocaml/opam/issues/4327` \
-      opam init --yes --disable-sandboxing && \
+      opam init --yes && \
       opam install --yes oasis
 
 RUN apt-get install -y --no-install-recommends \