Variable expansion in RUN --mount command does not work

[dln22]

I am trying to mount a source dir for one of my run commands in the docker file. Ideally, I would like to pass source dir as build-arg when calling docker build. However, I have noticed that the passed source dir is not correctly expanded. Here is a minimum dockerfile example reproducing the problem:

FROM alpine:latest
ARG source_dir
RUN --mount=type=bind,target=/indomain,source=${source_dir} 

and then I call it as follows:

docker buildx build -f Dockerfile --build-arg source_dir=./data .

and this is the error I get:

 => ERROR [2/2] RUN --mount=type=bind,target=/indomain,source=./data                                                                                                                         0.0s
------
 > [2/2] RUN --mount=type=bind,target=/indomain,source=./data:
------
error: failed to solve: failed to compute cache key: "/${source_dir}" not found: not found

It is a bit confusing as the first ERROR line shows the variable expanded, but then at the bottom, it shows it as not expanded.

According to the #2089 merge request, the variable expansion should be now supported?

[tonistiigi]

What version of dockerfile are you using? This was added in 1.3 https://github.com/moby/buildkit/releases/tag/dockerfile%2F1.3.0

[dln22]

This is the docker version output:

>docker version
Client:
 Cloud integration: v1.0.22
 Version:           20.10.12
 API version:       1.41
 Go version:        go1.16.12
 Git commit:        e91ed57
 Built:             Mon Dec 13 11:44:07 2021
 OS/Arch:           windows/amd64
 Context:           default
 Experimental:      true

Server: Docker Engine - Community
 Engine:
  Version:          20.10.12
  API version:      1.41 (minimum version 1.12)
  Go version:       go1.16.12
  Git commit:       459d0df
  Built:            Mon Dec 13 11:43:56 2021
  OS/Arch:          linux/amd64
  Experimental:     false
 containerd:
  Version:          1.4.12
  GitCommit:        7b11cfaabd73bb80907dd23182b9347b4245eb5d
 runc:
  Version:          1.0.2
  GitCommit:        v1.0.2-0-g52b36a2
 docker-init:
  Version:          0.19.0
  GitCommit:        de40ad0

[dln22]

Hmm I am not too familiar with different dockerfile versions. I have tried to add the # syntax line to my dockerfile:

# syntax=docker.io/docker/dockerfile:1.3.0
FROM alpine:latest
ARG source_dir
RUN --mount=type=bind,target=/indomain,source=${source_dir} 

but that resulted in another error:

#1 [internal] load build definition from Dockerfile
#1 transferring dockerfile: 2B 0.2s done
#1 DONE 0.3s
error: failed to solve: failed to solve with frontend dockerfile.v0: failed to read dockerfile: open /var/lib/docker/tmp/buildkit-mount852294124/Dockerfile: no such file or directory

[dln22]

ok, I got it working! Issue can be closed. Thanks for help @tonistiigi

[sfariaNG]

For future reference if anyone else runs into this issue, add the following as the very first line in the Dockerfile, before anything else including comments:
# syntax=docker.io/docker/dockerfile:1.3.0

Tested on:

 Plugins:
   buildx: Docker Buildx (Docker Inc., v0.9.1)
...
 Server Version: 20.10.21