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

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

If you are using sudo for docker commands, you might need:

sudo DOCKER_BUILDKIT=1 ...

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.

tl;dr

Dockerfile

# syntax=docker/dockerfile:experimental
FROM continuumio/miniconda3

RUN --mount=type=ssh pip install git+ssh://[email protected]/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
    }
}