I never said you can’t do that, I’m merely pointing out that GP’s example isn’t an example of doing that.
My point is that the Dockerfile is not a particularly good build system. It doesn’t solve any of the hard parts (how to build the thing, what to build when) and only solves problems it created (how to avoid re-running something on every invocation) and does it poorly (it doesn’t track dependencies, just determines cached-not-cached on every line.) It leads to awful hacks like putting every build step on an enormous chain of `cmd && \` lines to make sure you don’t produce extraneous layers.
A reasonable build system tracks dependencies for what needs to be built and has a target it’s trying to get to. Docker just says “do your build imperatively and we’ll figure out caching” and it’s extremely easy to shoot yourself in the foot leading to overly large images.
An example: there’s no way to say “build container: do this build and put the product here, runtime container: take the product and store it here” without creating a temporary layer in the build container that is not used anywhere else. So if your artifact is 1GB, you will always require 2GB to do a build: one for the runtime container (where you want it) and one for the build container (where you only needed it temporarily.) You just have to do “docker builder prune” every now and then when your disk inevitably gets full, there’s no way to simply not produce the duplicate in the first place.
(My workaround is to keep the artifact in a type=cache mount, store its SHA sum in the build container, then in the runtime container, grab the artifact from the type=cache mount and compare the SHA sum to make sure it didn’t change. Cache mounts can be GC’d at any time so it’s not even guaranteed to work.)
When I used docker at scale back in the day, the thing that worked best was always: bind-mount an artifacts dir into the build container, run the build. Then package up the artifacts dir. Then at runtime, fetch the artifacts, bind mount them into the runtime container. Then you can use a real artifact store on a plain HTTP server instead of messing with docker layers (no more worrying about pushing 100 1KB layers because you didn’t put enough &&’s in your dockerfile) and it was so, so much faster than any docker registry was in practice.
Dockerfiles are great for simple stuff but, if I already have a great build system that produces single binaries, it’s so much more pleasant to just use that.
My point is that the Dockerfile is not a particularly good build system. It doesn’t solve any of the hard parts (how to build the thing, what to build when) and only solves problems it created (how to avoid re-running something on every invocation) and does it poorly (it doesn’t track dependencies, just determines cached-not-cached on every line.) It leads to awful hacks like putting every build step on an enormous chain of `cmd && \` lines to make sure you don’t produce extraneous layers.
A reasonable build system tracks dependencies for what needs to be built and has a target it’s trying to get to. Docker just says “do your build imperatively and we’ll figure out caching” and it’s extremely easy to shoot yourself in the foot leading to overly large images.
An example: there’s no way to say “build container: do this build and put the product here, runtime container: take the product and store it here” without creating a temporary layer in the build container that is not used anywhere else. So if your artifact is 1GB, you will always require 2GB to do a build: one for the runtime container (where you want it) and one for the build container (where you only needed it temporarily.) You just have to do “docker builder prune” every now and then when your disk inevitably gets full, there’s no way to simply not produce the duplicate in the first place.
(My workaround is to keep the artifact in a type=cache mount, store its SHA sum in the build container, then in the runtime container, grab the artifact from the type=cache mount and compare the SHA sum to make sure it didn’t change. Cache mounts can be GC’d at any time so it’s not even guaranteed to work.)
When I used docker at scale back in the day, the thing that worked best was always: bind-mount an artifacts dir into the build container, run the build. Then package up the artifacts dir. Then at runtime, fetch the artifacts, bind mount them into the runtime container. Then you can use a real artifact store on a plain HTTP server instead of messing with docker layers (no more worrying about pushing 100 1KB layers because you didn’t put enough &&’s in your dockerfile) and it was so, so much faster than any docker registry was in practice.
Dockerfiles are great for simple stuff but, if I already have a great build system that produces single binaries, it’s so much more pleasant to just use that.