CI: use compatible options for diff.

Also install `ocp-indent` and embeds `.ocp-indent` in the docker
image.

scripts/Dockerfile.build.in

@@ -3,7 +3,7 @@ FROM $base_image
 COPY src tezos/src
 COPY test tezos/test
 COPY scripts tezos/scripts
-COPY Makefile jbuild tezos.opam tezos/
+COPY Makefile jbuild .ocp-indent tezos/
 
 RUN sudo chown -R opam /home/opam/tezos && \
     cd tezos && \

scripts/Dockerfile.build_deps.in

@@ -4,6 +4,7 @@ COPY scripts/install_build_deps.sh scripts/version.sh tezos/scripts/
 COPY tezos.opam tezos/
 RUN sudo apk update && \
     opam config exec -- ./tezos/scripts/install_build_deps.sh && \
+    opam install ocp-indent && \
     sudo rm -f /var/cache/apk/* && \
     rm -fr ~/.opam/log/ && \
     rm -fr "$(opam config exec -- ocamlfind query stdlib)"/topdirs.cmi

scripts/test-ocp-indent.sh

@@ -10,7 +10,7 @@ for f in  ` find \( -name _build -or \
                  \( -name \*.ml -or -name \*.mli \) -print`; do
   ff=$(basename $f)
   ocp-indent $f > $tmp_dir/$ff
-  diff -u --color $f $tmp_dir/$ff
+  diff -U 3 $f $tmp_dir/$ff
   if [ $? -ne 0 ]; then
       failed=yes
   fi