Error response from daemon: Dockerfile parse error Unknown flag: mount - docker

Question

There is a previous question (Docker Unknown flag --mount) facing the same error that was due to having an out-of-date version of Docker running. I have an up-to-date version of Docker running.
I have the following Dockerfile:
FROM continuumio/miniconda3
RUN --mount=type=ssh pip install git+ssh://git#github.com/myrepo/myproject.git#develop
RUN conda install numpy
...
According to the documentation, I should be able to simply run docker build --ssh default .. However, I receive the following error:
Sending build context to Docker daemon 2.048kB
Error response from daemon: Dockerfile parse error line 3: Unknown flag: mount
Output of docker version:
Client: Docker Engine - Community
Version: 18.09.2
API version: 1.39
Go version: go1.10.8
Git commit: 6247962
Built: Sun Feb 10 04:12:39 2019
OS/Arch: darwin/amd64
Experimental: false
Server: Docker Engine - Community
Engine:
Version: 18.09.2
API version: 1.39 (minimum version 1.12)
Go version: go1.10.6
Git commit: 6247962
Built: Sun Feb 10 04:13:06 2019
OS/Arch: linux/amd64
Experimental: true
I would like to build a Docker image without exposing my private SSH credentials, and this seemed to be the supported method. Anyone have thoughts on what's causing the issue?

Answer 1

tl;dr
Dockerfile
# syntax=docker/dockerfile:experimental
FROM continuumio/miniconda3
RUN --mount=type=ssh pip install git+ssh://git#github.com/myrepo/myproject.git#develop
RUN conda install numpy
...
Note: the comment on the first line is required voodoo
Then build your docker image with:
DOCKER_BUILDKIT=1 docker build --ssh default -t my_image .
With this, you will be able to use the --mount option for the RUN directive in your Dockerfile.
Long answer
As found in the documentation here, ssh forwarding when building docker image is enabled only when using the BuildKit backend:
External implementation features
This feature is only available when using the
BuildKit
backend.
Docker build supports experimental features like cache mounts, build
secrets and ssh forwarding that are enabled by using an external
implementation of the builder with a syntax directive. To learn about
these features, refer to the documentation in BuildKit
repository.
For this you need Docker 18.09 (or later) and you also need to run the docker build command with the DOCKER_BUILDKIT=1 environment variable and start your Docker file with the following magic comment : # syntax=docker/dockerfile:experimental.
Also you can edit /etc/docker/daemon.json and add :
{
"experimental" : false,
"debug" : true,
"features": {
"buildkit" : true
}
}

Answer 2

If you are using sudo for docker commands, you might need:
sudo DOCKER_BUILDKIT=1 ...

Answer 3

To anyone out there that might be struggling with this sort of error: ensure that the first line of the file is '# syntax=docker/dockerfile:experimental'. This will work:
# syntax=docker/dockerfile:experimental
FROM golang:1.14.1 as builder
...
RUN --mount=type=ssh GOSUMDB=off go get -d -v ./...
BUT if you add a comment at the start of the file like so:
# SOME SILLY COMMENT HERE <--- this ostensibly innocent comment ruins everything!
# syntax=docker/dockerfile:experimental
FROM golang:1.14.1 as builder
...
RUN --mount=type=ssh GOSUMDB=off go get -d -v ./...
Then 'syntax=...' will not be taken into account because it's no longer on the very first line. This is why you get that weird error about 'mount' below! Gah!
Hopefully this will save a few hours from the lives of a few people.

Answer 4

The error message that you are getting due to writing --mount inside the Dockerfile. You have to enable Docker BuildKit first in order to use this syntax.
You can check all of the currently available build options through here

Answer 5

Locally I just needed DOCKER_BUILDKIT=1 docker build...
In TeamCity build pipeline my docker file still needed the magic line to overcome this issue. My TeamCity agent is running Docker version 19.03.9
So in 2022 this is still my conclusion:
Try the magic line mentioned at the top of your Docker file:
#syntax=docker/dockerfile:experimental
Use BUILDKIT. e.g. DOCKER_BUILDKIT=1
.. and in my context this is all for running the following line:
RUN --mount=type=ssh npm install "git+ssh://git#github.com:dra_____.git"
Maybe this will save some folks some time.