WIP: First attempt at propper docker multiarch

docker/ci-binaries-packaging.Dockerfile

@@ -1,3 +1,4 @@
+# syntax=docker/dockerfile:1
 # ---------------------------------------------------------------------------------------------------------
 # This Dockerfile is intended to be built as part of Conduit's CI pipeline.
 # It does not build Conduit in Docker, but just copies the matching build artifact from the build job.
@@ -33,7 +34,7 @@ LABEL org.opencontainers.image.created=${CREATED} \
       org.opencontainers.image.revision=${GIT_REF} \
       org.opencontainers.image.source="https://gitlab.com/famedly/conduit.git" \
       org.opencontainers.image.licenses="Apache-2.0" \
-      org.opencontainers.image.documentation="" \
+      org.opencontainers.image.documentation="https://gitlab.com/famedly/conduit" \
       org.opencontainers.image.ref.name=""
 
 # Standard port on which Conduit launches. You still need to map the port when using the docker command or docker-compose.
@@ -42,9 +43,16 @@ EXPOSE 6167
 # create data folder for database
 RUN mkdir -p /srv/conduit/.local/share/conduit
 
-# Copy the Conduit binary into the image at the latest possible moment to maximise caching:
-COPY ./conduit-x86_64-unknown-linux-musl /srv/conduit/conduit
+# Test if Conduit is still alive, uses the same endpoint as Element
 COPY ./docker/healthcheck.sh /srv/conduit/
+HEALTHCHECK --start-period=5s --interval=60s CMD ./healthcheck.sh
+
+# Copy the Conduit binary into the image at the latest possible moment to maximise caching:
+
+# depending on the target platform (e.g. "linux/arm/v7", "linux/arm64/v8", or "linux/amd64")
+# copy the matching binary into this docker image
+ARG TARGETPLATFORM
+COPY ./$TARGETPLATFORM /srv/conduit/conduit
 
 # Add www-data user and group with UID 82, as used by alpine
 # https://git.alpinelinux.org/aports/tree/main/nginx/nginx.pre-install
@@ -57,10 +65,6 @@ RUN set -x ; \
 RUN chown -cR www-data:www-data /srv/conduit
 RUN chmod +x /srv/conduit/healthcheck.sh
 
-
-# Test if Conduit is still alive, uses the same endpoint as Element
-HEALTHCHECK --start-period=5s --interval=60s CMD ./healthcheck.sh
-
 # Set user to www-data
 USER www-data
 # Set container home directory