arm64 support with grub-efi

* build / convert: add `--platform` flag to support linux/amd64 &
linux/arm64
* build: add `--pull` flag
* run/hetzner: add `--type` flag to select server type
* run/hetzner: add `--location` flag to select server location

Signed-off-by: Adphi <philippe.adrien.nousse@gmail.com>

Dockerfile

@@ -38,7 +38,7 @@ RUN apt-get update && \
         dosfstools \
         mount \
         tar \
-        extlinux \
+        "$([ "$(uname -m)" = "x86_64" ] && echo extlinux)" \
         cryptsetup-bin \
         qemu-utils && \
     apt-get clean && \