dockerfile: Install the zip tool in the base image

This is needed when packaging the cross built toolchains anyway.

Dockerfile

@@ -3,7 +3,7 @@ FROM ubuntu:18.04
 RUN apt-get update -qq && apt-get install -qqy --no-install-recommends \
     git wget bzip2 file unzip libtool pkg-config cmake build-essential \
     automake yasm gettext autopoint vim-tiny python3 python3-distutils \
-    ninja-build ca-certificates curl less && \
+    ninja-build ca-certificates curl less zip && \
     apt-get clean -y && \
     rm -rf /var/lib/apt/lists/*