Use multi-stage build

[taliesins]

To minimize the size of the image we probably want to build haproxy in one container and then copy the build outputs into the next container. This way the build tools are not in the haproxy container (smaller/safer image).

https://docs.docker.com/build/building/multi-stage/
https://github.com/b4tman/docker-squid/blob/master/Dockerfile

[yosifkit]

To which apt-installed dependencies are you referring to in the final image? The Dockerfile goes to great lengths to ensure that only packages that are linked to are kept after building haproxy, so things like gcc and make are definitely removed:

haproxy/3.0/Dockerfile

Lines 82 to 92 in 8db4f30

	apt-mark auto '.*' > /dev/null; \
	[ -z "$savedAptMark" ] || apt-mark manual $savedAptMark; \
	find /usr/local -type f -executable -exec ldd '{}' ';' \
	| awk '/=>/ { so = $(NF-1); if (index(so, "/usr/local/") == 1) { next }; gsub("^/(usr/)?", "", so); printf "*%s\n", so }' \
	| sort -u \
	| xargs -r dpkg-query --search \
	| cut -d: -f1 \
	| sort -u \
	| xargs -r apt-mark manual \
	; \
	apt-get purge -y --auto-remove -o APT::AutoRemove::RecommendsImportant=false; \