Dockerfile: install git-lfs from buster-backports.

Debian Buster comes with version 2.7.1 of git-lfs which does not support
git+ssh yet, therefore we need a newer version of git-lfs for cloning a
repository which uses LFS.

Signed-off-by: Steffen Hieber <steffen.hieber@siemens.com>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>

Dockerfile

@@ -5,7 +5,8 @@ FROM debian:buster-slim
 ARG TARGETPLATFORM
 ARG DEBIAN_FRONTEND=noninteractive
 
-RUN apt-get update && \
+RUN echo 'deb http://deb.debian.org/debian buster-backports main' > /etc/apt/sources.list.d/buster-backports.list && \
+    apt-get update && \
     apt-get install -y locales && \
     localedef -i en_US -c -f UTF-8 -A /usr/share/locale/locale.alias en_US.UTF-8
 ENV LANG=en_US.utf8
@@ -18,7 +19,7 @@ RUN apt-get install --no-install-recommends -y \
         python3-setuptools python3-wheel python3-yaml python3-distro python3-jsonschema \
         gosu lsb-release file vim less procps tree tar bzip2 zstd bc tmux libncurses-dev \
         dosfstools mtools parted \
-        git-lfs mercurial iproute2 ssh-client curl rsync gnupg awscli sudo && \
+        git-lfs/buster-backports mercurial iproute2 ssh-client curl rsync gnupg awscli sudo && \
     if [ "$TARGETPLATFORM" = "linux/amd64" ]; then \
         apt-get install --no-install-recommends -y gcc-multilib syslinux; \
     fi && \