Docker image is completely flat

[fatalbanana]

Docker image is completely flat

[polarathene]

20.10 point release could support it with the syntax hint at the top of the Dockerfile. In this comment you referred to the version of Docker as 2020, not sure if you are using a very early release of 20.10 or newer one that can use DOCKER_BUILDKIT=1.

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Is it necessary to use Drone? Github Actions are well suited for this and I'm happy to explain the ones used for docker-mailserver if you're interested in adopting something similar.

Looking at the drone config, one downside to Github Actions is ARM64 has no native runners, you need to emulate those builds via QEMU (this is done easily, but it's much slower).

You could have Github Actions delegate to self-hosted runners on your own hardware for native ARM64 builds though, but then I guess you'd run into the same issue with outdated Docker Engine version 😅

[polarathene]

For additional reference, since you were not able to adopt RUN --mount=type=bind to avoid a COPY you don't need to persist, when you are able to leverage this, I provided a snippet here which you could also throw in --mount=type=cache for apt to use (both mount types can be used with the single RUN).

For your Drone systems this may be a non-issue, but on ephemeral CI runners like Github Actions where the environment is reset each run, cache from CI runs is persisted remotely. Docker buildx can export the build image layers as cache, but cache mounts are a different kind, they need additional handling to restore for future use.

On local dev systems where that's a non-issue, the cache mount is convenient as the data is not lost to invalidated layers. You can alternatively use it without persistence to keep data out of the image, but then the tmpfs mount is probably more suitable.