Check for curl as a dependency for export-image (#95)

* Check for curl as a dependency for export-image

* Add curl to the Dockerfile

Dockerfile

@@ -4,7 +4,7 @@ RUN apt-get -y update && \
     apt-get -y install \
         git vim parted \
         quilt realpath qemu-user-static debootstrap zerofree pxz zip dosfstools \
-        bsdtar libcap2-bin rsync grep udev xz-utils \
+        bsdtar libcap2-bin rsync grep udev xz-utils curl \
     && rm -rf /var/lib/apt/lists/*
 
 COPY . /pi-gen/

depends

@@ -11,3 +11,4 @@ bsdtar
 grep
 rsync
 xz:xz-utils
+curl