fix(docker): stamp version metadata into the published image

[thomasboni]

Fixes #425

Problem

plumber version inside the published Docker image reports:

plumber version dev
  commit: none
  built:  unknown

while the downloaded release binaries report the real version.

Root cause

release.yml stamps the release binaries with -X github.com/getplumber/plumber/cmd.Version=... / cmd.Commit=... / cmd.BuildDate=... ldflags, but the docker job builds the image straight from the Dockerfile, whose go build only passes -s -w. The binary in the image therefore keeps the compile-time defaults from cmd/version.go (dev / none / unknown).

Fix

• Dockerfile: add VERSION / COMMIT / BUILD_DATE build args (defaulting to the current dev values) and feed them into the same -X ldflags the release binaries use. The args are declared after the dependency layers so a changing BUILD_DATE never invalidates the go mod download cache.
• release.yml (docker job): pass the three build args to docker/build-push-action, with COMMIT=${{ github.sha }} matching what the release binaries embed and BUILD_DATE computed in the same format as the binary build step.

Plain docker build with no args (ci.yml grype scan, local builds) keeps today's dev output.

Verification

Built the image locally from this branch:

• with --build-arg VERSION=9.9.9-test --build-arg COMMIT=abc1234def --build-arg BUILD_DATE=2026-08-21T00:00:00Z:

  plumber version 9.9.9-test
    commit: abc1234def
    built:  2026-08-21T00:00:00Z
  

• with no build args (ci.yml path): unchanged dev / none / unknown defaults.

The published image will pick this up at the next release.

🤖 Generated with Claude Code