Merge pull request #2197 from txt-file/patch-2

 contrib/Dockerfile: use apt-get instead of apt

contrib/Dockerfile

 FROM debian:buster-slim
 
-RUN apt update && apt install -y --no-install-recommends \
+ARG DEBIAN_FRONTEND=noninteractive
+RUN apt-get update && apt-get install -y --no-install-recommends \
     ca-certificates \
     file \
     git \