most convenient way to test a file for a provider?

[gcb]

for a small team, who just want to test their scripts will mostly work, without spinning hundreds of vms on their cloud provider. what is the most convenient way to test cloud-init files?

my current setup is to

1. start a docker container with the same base image as used by my provider (e.g. ubuntu:stable, debian:13, etc), and install cloud-init package.
2. copy my user-data.yml file
3. run cloud-init schema -c user.yaml --annotate

this is enough to give me a OK if there are any obvious errors, but i'd like to take to the next step and actually execute the file. I'm aware of having to deal with side effects and that's ok, but I wish i could use the convenience of disposable docker instances and being already logged in the machine before execution so i can inspect it more easily. (something like cloud-init init --local --file user.yml but without all the errors one can expect such as missing network)

i'd like to avoid having to deal with meta-data/vendor files, temporary webservers, local dns hacks, qemu, etc. Is this possible?

[gcb]

for the record, this is what we use today, and the experience is awful.

default: clean build

.PHONY: clean
clean:
        -rm root.qcow2

debian-original.qcow2:
        curl -o $@ -L https://cloud.debian.org/images/cloud/trixie/latest/debian-13-genericcloud-amd64.qcow2

root.qcow2: debian-original.qcow2
        cp $< $@

seed-www.iso: src/www.yaml meta-data
        touch network-config
        touch meta-data
        cp src/www.yaml user-data
        genisoimage -output $@ -volid cidata -rational-rock -joliet user-data meta-data network-config

.PHONY: build
build: seed-www.iso root.qcow2
        qemu-system-x86_64 -m 1024 -net nic -net user \
        -netdev user,id=net0,hostfwd=tcp::2222-:22 \
        -device virtio-net-pci,netdev=net0 \
        -hda root.qcow2 \
        -cdrom seed-www.iso &

then if at least the minimal user/keys/ssh/etc steps worked, we can ssh to the qemu vm and inspect /var/log/cloud-init.

[holmanb]

what is the most convenient way to test cloud-init files? ... I wish i could use the convenience of disposable docker instances and being already logged in the machine before execution so i can inspect it more easily. (something like cloud-init init --local --file user.yml but without all the errors one can expect such as missing network)

i'd like to avoid having to deal with meta-data/vendor files, temporary webservers, local dns hacks, qemu, etc. Is this possible?

I would use LXD. This allows using different distros, running an lxc container locally, login during early boot, and no need to run any commands manually. Just pass the user-data, vendor-data, and meta-data as arguments on launch then execute cloud-init status --wait --long in the container and check for errors once complete.