Exclude .deb files from release Docker image (#5068)

ci/release-image/Dockerfile

+# syntax=docker/dockerfile:experimental
+
+FROM scratch AS packages
+COPY release-packages/code-server*.deb /tmp/
+
 FROM debian:11
 
 RUN apt-get update \
@@ -34,9 +39,8 @@ RUN ARCH="$(dpkg --print-architecture)" && \
     mkdir -p /etc/fixuid && \
     printf "user: coder\ngroup: coder\n" > /etc/fixuid/config.yml
 
-COPY release-packages/code-server*.deb /tmp/
 COPY ci/release-image/entrypoint.sh /usr/bin/entrypoint.sh
-RUN dpkg -i /tmp/code-server*$(dpkg --print-architecture).deb && rm /tmp/code-server*.deb
+RUN --mount=from=packages,src=/tmp,dst=/tmp/packages dpkg -i /tmp/packages/code-server*$(dpkg --print-architecture).deb
 
 EXPOSE 8080
 # This way, if someone sets $DOCKER_USER, docker-exec will still work as