Building one image to deploy to multiple destinations

[mudge]

We currently use Kamal to continuously deploy an application to two destinations: staging and production.

We do this by using kamal deploy -d staging and kamal deploy -d production in a CI workflow but this causes the same Docker image to be built twice: once for each destination even though the resulting images are identical (as there are no per-environment build secrets).

Is there a recommended way to build an image once but then deploy it to separate destinations given that kamal deploy -d will tag images with their destination (e.g. latest-staging) or does it only really support building and tagging images per destination?

[jokesterfr]

Accordingly to the doc (https://kamal-deploy.org/docs/configuration/builders/) kamal uses the docker build system. I would recommend to download the built image (in staging) to rebuild the production image, with the build-cache (https://kamal-deploy.org/docs/configuration/builders/#builder-cache) you would get this build performed almost instantaneously.

TLDR; docker has built-in solution for your problem with a build cache mechanism.

[morgoth]

We're doing exactly that: build one image for staging deploy, and then reuse this image for production deploy.

You can deploy existing docker image via kamal deploy --skip-push. It will not build the image but will try to fetch already built one.