Split Dockerfile in two stages: builder and runtime.

[pablospe]

Splitting the Dockerfile in two stages reduces the size of the final image considerably, since all the build dependencies are not needed.

You can still create the tag for the stages separately:

docker build --target builder -t colmap:builder .
docker build --target runtime -t colmap:runtime .

But the default command (without --target) is the last stage, so:

docker build -t colmap:latest .

will refer to --target runtime because it is the last stage in the Dockerfile.

[ahojnnes]

Thanks, great improvement.

[pablospe]

FYI, this PR reduces the docker image ~3GB :)

(image from https://hub.docker.com/r/colmap/colmap)