variables not expanded with buildkit + too many build stages executed

[laur89]

Attempting to trigger only specific intermediate build stages (step-two in our example), but arg DEFINED_IN_SECOND_STEP defined in intermediary step is not visible to our final stage, as witnessed by logged out "in final stage FINAL_ARG=[-extra-value]" message.

Additionally, both step-one & step-two stages are executed, that should not happen.

drone.yml:

kind: pipeline
type: docker
name: default

steps:
- name: publish-image
  image: plugins/docker
  settings:
    target: final
    build_args:
    - COMMON_STEP_VAR=common-step-value
    - OPTIONAL_STEP=two  # one | two
    dry_run: true
    debug: true

trigger:
  ref:
  - refs/heads/master
  - refs/heads/develop
  - refs/heads/feature/*
  - refs/tags/*

Dockerfile:

ARG OPTIONAL_STEP

FROM alpine:3 AS base

ARG COMMON_STEP_VAR=common-step-default
RUN echo "start, COMMON_STEP_VAR=[$COMMON_STEP_VAR]"

FROM base AS step-one
ARG DEFINED_IN_SECOND_STEP=step-one-${COMMON_STEP_VAR}
RUN echo "in stage one,  DEFINED_IN_SECOND_STEP=[$DEFINED_IN_SECOND_STEP]"


FROM base AS step-two
ARG DEFINED_IN_SECOND_STEP=step-two-${COMMON_STEP_VAR}
RUN echo "in stage two,  DEFINED_IN_SECOND_STEP=[$DEFINED_IN_SECOND_STEP]"


FROM step-${OPTIONAL_STEP} AS final
ARG FINAL_ARG=${DEFINED_IN_SECOND_STEP}-extra-value
RUN echo "in final stage  FINAL_ARG=[$FINAL_ARG]"

---

Running

$ docker build --rm=true -f Dockerfile -t deleteme . --pull=true \
    --build-arg COMMON_STEP_VAR=common-step-value \
    --build-arg OPTIONAL_STEP=two \
    --target final 

locally however logs out the expected

CACHED [final 1/1] RUN echo "in final stage FINAL_ARG=[step-two-common-step-value-extra-value]"

Also step-one is not executed as expected.