Interpolating strings in a file path in a DockerFile - azure-devops

Question

I have a Docker file which starts like this:
ARG FILE_PATH
FROM mcr.microsoft.com/dotnet/aspnet:3.1 AS base
WORKDIR /app
EXPOSE 80
EXPOSE 443
FROM mcr.microsoft.com/dotnet/sdk:3.1 AS build
WORKDIR /src
COPY ["${FILE_PATH}/src/NuGet.config", "src/"]
I call it using the azure-cli like this:
$pathToSrc = "$(Build.SourcesDirectory)/My folder"
az acr build --build-arg "FILE_PATH=$pathToSrc" ...
This always fails with the message:
COPY failed: file not found in build context or excluded by
.dockerignore: stat src/NuGet.config: file does not exist
I have tried variations such as:
COPY [$FILE_PATH/src/NuGet.config, "src/"]
COPY ["FILE_PATH/src/NuGet.config", "src/"]
and
az acr build --build-arg "FILE_PATH='$pathToSrc'" ...
but always end up with the same message.
Is there a way to do this. I am running on a hosted agent in Azure-devops pipeline. The task is task: AzureCLI#2 using a PowerShell Core script.

Answer 1

This may be related: https://stackoverflow.com/a/56748289/4424236
...after every FROM statements all the ARGs gets collected and are no longer available. Be careful with multi-stage builds.
Try this:
FROM mcr.microsoft.com/dotnet/aspnet:3.1 AS base
WORKDIR /app
EXPOSE 80
EXPOSE 443
FROM mcr.microsoft.com/dotnet/sdk:3.1 AS build
WORKDIR /src
ARG FILE_PATH
COPY ["${FILE_PATH}/src/NuGet.config", "src/"]