Docker build with -f option cannot find Dockerfile

Question

I have tried a lot of combinations with the docker -f option but I have never got it to work

I can do this:

docker build -t foo/bar .

But I cannot do this:

docker build -t foo/bar -f Dockerfile .

or this:

docker build -t foo/bar -f ./Dockerfile .

This gives me the following error:

unable to prepare context: The Dockerfile (c:\path\Dockerfile) must be within the build context (.)

I am using docker through the default vm on Windows 7. Docker version is 1.8.1, build d12ea79

I cannot see the difference. It could be very nice to have different Dockerfiles for different tasks on a project, but without the -f option that is really not possible.

Answer 1

Well, as the error message is suggesting, your Dockerfile is not within the context, which is the current directory (.) in your case.

The docker file, specified with -f, must always be within the context directory specified as an argument.

So, normally this should work fine:

docker build -f /path/to/context/dir/Dockerfile /path/to/context/dir

And this too:

cd /some/dir
docker build -f /some/dir/customDir/Custom-Dockerfile-name .

While, this would fail:

docker build -f /path/to/diff/dir/Dockerfile /path/to/context/dir

From the Docs:

docker build [OPTIONS] PATH | URL | -

The docker build command builds Docker images from a Dockerfile and a “context”. A build’s context is the set of files located in the specified PATH or URL

And:

By default the docker build command will look for a Dockerfile at the root of the build context. The -f, --file, option lets you specify the path to an alternative file to use instead. This is useful in cases where the same set of files are used for multiple builds. The path must be to a file within the build context. If a relative path is specified then it is interpreted as relative to the root of the context.

Answer 2

Here is an example of how I used the docker build to avoid this error

docker build -f my.dockerfile ./

Note the trailing forward slash