Use multi-stage build

[buchdag]

https://docs.docker.com/develop/develop-images/multistage-build/

This Dockerfile does not fetch a pre built binary of docker-gen like the current one but compile it from source during the first stage, then grab the compiled binary for the final image from this stage.

This allows to build the image on any arch the golang and alpine images are available for from a single Dockerfile.

Tested and working on arm 32 bits arch (Raspbian running on a Raspberry Pi 3).

I feel this is a cleaner and more flexible solution than the usual method of maintaining a different Dockerfile for each arch (as proposed in #372). The drawbacks are:

• requires Docker 17.05+ to build
• the intermediate stage containers have to be manually cleaned

[wiget]

Thanks @buchdag for upstreaming this idea. I tested this on HypriotOS/aarch64 on Raspberry Pi 3 B+

[rriemann]

@buchdag , do you maintain a fork we can use until your changes are merged?

[buchdag]

This PR has already been merged 7 months ago :)

[rriemann]

@buchdag , oh my bad - that's true.

Anyway, I still run into a problem when I have the companion as a dependency of Nextcloud on an Scaleway ARM server. I use this docker-compose file: https://github.com/nextcloud/docker/blob/master/.examples/docker-compose/with-nginx-proxy/postgres/fpm/docker-compose.yml

The composer log suggests that arm is not supported:

standard_init_linux.go:190: exec user process caused "exec format error"

[buchdag]

Do you build the image from sources on the Scaleway ARM server ?

[rriemann]

I just called docker-compose build --push and docker-compose up -d.

[buchdag]

The goal of this PR was to allow building the image from the Dockerfile on any arch, not providing multi arch image straight from Dockerhub. The image you pull from Dockerhub is still amd64 only.

Clone the repo on your Scaleway server and build the image from it. It does not work with docker-compose build but with docker build -t jrcs/letsencrypt-nginx-proxy-companion . from inside the cloned repo (BTW I'm not aware of any --push flag to docker-compose build).