support/docker: add curl

Commit afece24a72 ("support/download: introduce curl backend for FTP
transfers") added curl download support, so add it to the container as well.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

support/docker/Dockerfile

@@ -28,6 +28,7 @@ RUN apt-get -o APT::Retries=3 install -y --no-install-recommends \
         ca-certificates \
         cmake \
         cpio \
+        curl \
         cvs \
         file \
         flake8 \