fixes archlinux docker image (#1206)

After we added `conf-radare2` dependency, the `bap` installation requires `radare2` to present in the system.
Unfortunately, `opam-depext.1.1.3` doesn't properly support `archlinux`,
so we have to install `radare2` manually.

Author: gitoleg

docker/archlinux/Dockerfile

@@ -10,7 +10,7 @@ WORKDIR /home/bap
 
 RUN sudo pacman -S --noconfirm m4 git unzip make curl wget \
          diffutils patch make gcc pkgconfig llvm python2 \
-         which clang \
+         which clang radare2 \
   && wget https://raw.githubusercontent.com/ocaml/opam/master/shell/install.sh \
   && echo "" | sudo sh install.sh \
   && opam init --comp=4.09.1 --disable-sandboxing --yes \