Don't use apt-key, it's deprecated.

Inspired by
https://michael-prokop.at/blog/2021/02/16/how-to-properly-use-3rd-party-debian-repository-signing-keys-with-apt/
and https://wiki.debian.org/DebianRepository/UseThirdParty. Also use the
newer deb822 sources format.

sogo/.dockerignore

 *
+!inverse.sources

sogo/Dockerfile

+FROM registry.hub.docker.com/library/debian:buster-slim as repo-key
+# hadolint ignore=DL3008
+RUN apt-get update && \
+    DEBIAN_FRONTEND=noninteractive apt-get install -y --no-install-recommends \
+        ca-certificates \
+        curl \
+        gnupg \
+    && \
+    rm -rf /tmp/* /var/tmp/* /var/lib/apt/lists/* /var/cache/apt/archives/*
+WORKDIR /gnupghome
+ENV GNUPGHOME /gnupghome
+RUN gpg --keyserver hkp://keys.gnupg.net --recv-key 0x810273C4 && \
+    gpg --output inverse.gpg --export-options=export-minimal --export 0x810273C4
+
 FROM registry.hub.docker.com/library/debian:buster-slim
 # hadolint ignore=DL3008
 RUN apt-get update && \
     DEBIAN_FRONTEND=noninteractive apt-get install -y --no-install-recommends \
         apt-transport-https \
         ca-certificates \
-        gnupg \
     && \
-    apt-key adv --keyserver hkp://keys.gnupg.net --recv-key 0x810273C4 && \
-    echo 'deb https://packages.inverse.ca/SOGo/nightly/5/debian/ buster buster' > /etc/apt/sources.list.d/sogo.list && \
-    apt-get update && \
+    rm -rf /tmp/* /var/tmp/* /var/lib/apt/lists/* /var/cache/apt/archives/*
+COPY --from=repo-key /gnupghome/inverse.gpg /usr/share/keyrings/
+COPY inverse.sources /etc/apt/sources.list.d/
+# hadolint ignore=DL3008
+RUN apt-get update && \
     DEBIAN_FRONTEND=noninteractive apt-get install -y --no-install-recommends \
         sogo-activesync \
         sogo \

sogo/inverse.sources

+Types: deb
+URIs: https://packages.inverse.ca/SOGo/nightly/5/debian/
+Suites: buster
+Components: buster
+Signed-By: /usr/share/keyrings/inverse.gpg