Build args should be preserved during the livepush

[CameronDiver]

This is possible by looking at the image history. If we look at the first entry into the history array returned for an image (we call it once per stage) we can then parse out the values of the build arguments which created this layer, and apply it to every step in the stage. We may not even need to parse the arguments and instead send them directly to the shell which is executing the update command.

An extension to this would be to find the layer we are currently executing in the history list, and only apply the arguments which were defined at that point.

[DaneSlattery]

I could definitely use this. In this example Dockerfile.template:

FROM balenalib/%%BALENA_ARCH%%-debian-dotnet:7.0-sdk as sdk

ARG BUILD_CONFIGURATION=Release

WORKDIR /usr/src/app
COPY ./ConsoleApp1 ./ConsoleApp1
RUN dotnet publish ConsoleApp1/ConsoleApp1.csproj -c ${BUILD_CONFIGURATION} -o publish

FROM balenalib/%%BALENA_ARCH%%-debian-dotnet:7.0-run as run

WORKDIR /usr/src/app

COPY --from=sdk /usr/src/app/publish .

CMD [ "dotnet", "ConsoleApp1.dll"]

When I change some code in the ConsoleApp project, dotnet publish step fails because the $BUILD CONFIGURATION argument is not available.