`compose build --dry-run` performs a real build and leaves a tagged image

[KennethWKZ]

mocker version: 0.7.2 (Homebrew) — macOS, Darwin 25.5.0, arm64

Summary

mocker compose build --dry-run executes a real build. BuildKit runs, layers are exported, and a tagged image is left in the image store. --dry-run is documented as "Execute command in dry run mode" and must not mutate state.

Reproduction

Any service with a build: section:

$ mocker compose -f flat.yml -p demo --project-directory /abs/path build --dry-run caddy
#7 exporting layers 0.0s done
#7 exporting manifest sha256:4431f16a57ab8ed33dd57a675bcd9f44c81c527bf4a776475e0e2f9d6b071d36 done
#7 exporting config sha256:3d11c086bab444a9b01c9cd333cae8f97a6b28bebd0ead0771eed684670dfc88 done
#7 exporting manifest list sha256:e2fa1a8d5a731b594df9fa23e705a47eced3419232494bccf049d94aacad67d3 done
#7 sending tarball
#7 DONE 0.4s
caddy:latest
Building caddy...
Successfully built caddy

The image exists afterwards:

$ mocker images | grep -i caddy
caddy   latest   sha256:e2fa1   in 0 sec   Zero KB

Expected

--dry-run prints the actions that would be taken and exits without invoking BuildKit or writing any image. Upstream docker compose build --dry-run reports the planned build steps only.

Worth auditing the flag across the other subcommands that advertise it (up, down, create, pull, run, stop, rm) — if it is a no-op parse for build, the others may accept and ignore it too. A silently-ignored --dry-run is worse than an unsupported-flag error, since --dry-run is the flag people reach for specifically to avoid touching anything.

Found while testing a build; the tagging behavior in that same output is filed separately as #73.

[us]

fixed in #77, shipped in v0.8.0.

you were right to ask for the audit — the flag was declared 35 times and read zero times. it's now declared once on the shared compose options and honored by every mutating subcommand, which prints the actions it would take and returns before touching any manager, engine or directory (the guard sits ahead of ensureDirectories too, so a dry run really writes nothing). it also works in docker's position, mocker compose --dry-run up.

compose build --dry-run now leaves no image behind.