And if you want something weird that's not supported by your particular tool of choice, you have the escape hatch of running arbitrary commands in the Dockerfile.
What more do you want?
I'd get your worry if we were talking about splitting up a terraform config and running it across multiple RUN directives, but we're not, are we?
Random examples off the top of my head: Puppet has a ton of transitive Ruby libraries and config files/caches that it leaves around; Terraform leaves some very big provider caches on the system; plan or output files, if generated and not cleaned up, can contain secrets; even the “control group” of the status quo with RUN instructions often results in package manager indexes and caches being left in images.
Those are all technically user error (hence why I called them footguns rather than defects), but they add up and are easy mistakes to make.
Until you need to do something that isn't covered with its DSL, and you extend it with an external command execution declaration... At which point people will just write bash scripts anyway and use your declarative language as a glorified exec.
In Azure YAML I had an odd bug because I used succeeded() instead of not(failed()) as a condition. I had no way of testing the pipeline without executing it. And each DSL has its own special set of sharp edges.
At least Bash's common edges are well known.
However, Dockerfiles are so popular because they run shell commands and permit 'socially' extending someone else shell commands; tacking commands onto the end of someone else's shell script is a natural process. /bin/sh is unreasonably effective at doing anything you need to a filesystem, and if the shell exposes a feature, it has probably been used in a Dockerfile somewhere.
Every other solution, especially declarative ones, tend to come up short when _layering_ images quickly and easily. However, I agree they're good if you control the entire declarative spec.
They sounded nice on paper but the work they replaced was somehow more annoying.
I moved over to Docker when it came out because it used shell.
I'd get much better results it I used something else to do the foreach and gave terraform only static rules.
Do you mean that if you use a dynamic output in a foreach, Terrafom can error? Or are you referring to “dynamic” blocks and their interactions with iterators?
If your dockerfile says “ensure package X is installed at version Y” that’s a lot clearer (and also more easy to make performant/cached and deterministic) than “apt-get update; apt-get install $transitive-at-specific-version; apt-get install $the-thing-you-need-atspecific-version”. I’m not thrilled at how distro-locked the shell version makes you, and how easy it is for accidental transitive changes to occur too.
But neither of those approaches is at a particularly low abstraction level relative to the OS itself; files and system calls are more or less hidden away in both package-manager-via-bash and puppet/terraform/whatever.
Its a Buildkit frontend, so you still use "docker build".
I wish we had standardized on something other than shell commands, though. Puppet or terraform or something more declarative would have been such a better alternative to “everyone cargo cults ‘RUN apt-get upgrade’ onto the top of their dockerfiles”.
Like, the layer/stage/caching behavior is fine. I just wish the actual execution parts had been standardized using something at a higher level of abstraction than shell.