RUNs between two FROMs never run

[AbhyudayaSharma]

Unlike the legacy builder, when using two FROMs in a Dockerfile, the RUN or COPY steps between them do not run. For example, consider the following Dockerfile:

FROM alpine:3.14.3 AS foo1
RUN sleep infinity
FROM alpine:3.15.0

When using DOCKER_BUILDKIT=0, the build never finishes until manually interrupted, as expected.

❯ DOCKER_BUILDKIT=0 docker build -t foo .
Sending build context to Docker daemon  2.048kB
Step 1/3 : FROM alpine:3.14.3 AS foo1
 ---> 0a97eee8041e
Step 2/3 : RUN sleep infinity
 ---> Running in 4c952c394f4c
^C

However, when using Buildkit, the build terminates immediately, not running the specified RUN command. As a result, the produced image is not what we wanted.

❯ DOCKER_BUILDKIT=1 docker build -t foo .
[+] Building 3.6s (5/5) FINISHED
 => [internal] load build definition from Dockerfile                                                               0.0s
 => => transferring dockerfile: 37B                                                                                0.0s
 => [internal] load .dockerignore                                                                                  0.0s
 => => transferring context: 2B                                                                                    0.0s
 => [internal] load metadata for docker.io/library/alpine:3.15.0                                                   3.5s
 => CACHED [stage-1 1/1] FROM docker.io/library/alpine:3.15.0@sha256:21a3deaa0d32a8057914f36584b5288d2e5ecc984380  0.0s
 => exporting to image                                                                                             0.0s
 => => exporting layers                                                                                            0.0s
 => => writing image sha256:35e223a20dbce8c0b81d3257f8cad0c7b2b35d8e18eadfec7eeb7de86a472e7b                       0.0s
 => => naming to docker.io/library/test                                                                            0.0s

---

Docker Version:

Client:
 Version:           20.10.7
 API version:       1.41
 Go version:        go1.13.8
 Git commit:        20.10.7-0ubuntu1~20.04.2
 Built:             Fri Oct  1 14:07:06 2021
 OS/Arch:           linux/amd64
 Context:           default
 Experimental:      true

Server: Docker Engine - Community
 Engine:
  Version:          20.10.8
  API version:      1.41 (minimum version 1.12)
  Go version:       go1.16.6
  Git commit:       75249d8
  Built:            Fri Jul 30 19:52:31 2021
  OS/Arch:          linux/amd64
  Experimental:     false
 containerd:
  Version:          1.4.9
  GitCommit:        e25210fe30a0a703442421b0f60afac609f950a3
 runc:
  Version:          1.0.1
  GitCommit:        v1.0.1-0-g4144b63
 docker-init:
  Version:          0.19.0
  GitCommit:        de40ad0

Docker config:

{
  "registry-mirrors": [],
  "insecure-registries": [],
  "debug": false,
  "experimental": false,
  "features": {
    "buildkit": true
  },
  "builder": {
    "gc": {
      "enabled": true,
      "defaultKeepStorage": "20GB"
    }
  }
}

I have only tested this inside Ubuntu 20.04 LTS in WSL2 on Windows 10 version 1909:

❯ lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description:    Ubuntu 20.04.3 LTS
Release:        20.04
Codename:       focal

OS Name:                   Microsoft Windows 10 Enterprise
OS Version:                10.0.18363 N/A Build 18363
OS Manufacturer:           Microsoft Corporation

[crazy-max]

@AbhyudayaSharma This is by design in BuildKit. Here the final stage you are building has no dependency on the first one so it's skipped. See also docker/build-push-action#377 (comment).