How to pass file as argument for a dockerfile

Question

Actually I'm not sure this can be done but I know there is always a hack , so I am enthusiastic about what you experts will say about this.

l want to pass a package as an argument when building my docker image so that I can deploy separate app versions with the same dockerfile.

This is an example for what I want to achieve :
The ability to pass package as an argument when building the image ( I know the syntax is wrong , it's just an example )

 docker build -t myapp/myapp:v1 . package_myapp_v1.tar  

Then use ADDin the Dockerfile to extract it into the container.

ADD passed_package ./ 

So that if I want to build an image for my app's v2 I can just specify the new package in docker build command without changing the Dockerfile.

Anyone knows what's the trick to get that working?

Thanks :)

Answer 1

I've managed to solve this after two minutes of reading in docker's doc.

The ARG instruction defines a variable that users can pass at build-time to the builder with the docker build command using the --build-arg = flag. If a user specifies a build argument that was not defined in the Dockerfile, the build outputs an error.

So I've used this to build my image

docker build -t myapp/v1 --build-arg package=package_myapp_v1.tar.gz .

Then to get the package inside Dockerfile use ARG instruction:

ARG package
ADD $package ./

The content of $package variable is package_myapp_v1.tar.gz which is passed through the tag package.

Answer 2

It seems like it is the context that should be changing between the builds, not the Dockerfile, right? Either way, one way to achieve this is to use Docker's ability to receive the context as a tar (optionally gz) file, with the Dockerfile at its root:

tar --create \
    /some/path/to/Dockerfile \
    /other/path/to/package-X.Y.Z.tgz \
    --transform 's,.*/,,' \
    --transform 's/package.*tgz/package.tgz/' |
docker build -t myapp/myapp:v1 -

Note:

• The first transform flattens the directory structure, so all files appear in the root of the tar file, where Docker expects them.

• The second transform removes the version number from the package file, so that you don't have to edit the ADD package.tgz . line in your Dockerfile.