The current operating system is not capable of running this task - azure-devops

Question

From an Azure DevOps pipeline, using a self hosted linux build agent running on docker, I get the error below in terraform plan. I tried many things, even running the steps from a bash shell on the build agent: this worked well.
Do you have any suggestion?
2019-09-12T13:55:21.8133489Z ##[debug]Evaluating condition for step: 'Terraform plan'
2019-09-12T13:55:21.8134075Z ##[debug]Evaluating: succeeded()
2019-09-12T13:55:21.8134246Z ##[debug]Evaluating succeeded:
2019-09-12T13:55:21.8134443Z ##[debug]=> True
2019-09-12T13:55:21.8134723Z ##[debug]Result: True
2019-09-12T13:55:21.8134976Z ##[section]Starting: Terraform plan
2019-09-12T13:55:21.8138406Z ==============================================================================
2019-09-12T13:55:21.8138526Z Task : Run Terraform
2019-09-12T13:55:21.8138605Z Description : Run a Terraform on the build agent
2019-09-12T13:55:21.8138647Z Version : 2.4.0
2019-09-12T13:55:21.8138688Z Author : Peter Groenewegen - Xpirit
2019-09-12T13:55:21.8138772Z Help : [More Information](https://pgroene.wordpress.com/2016/06/14/getting-started-with-terraform-on-windows-and-azure/)
2019-09-12T13:55:21.8138828Z ==============================================================================
2019-09-12T13:55:21.8347594Z ##[error]The current operating system is not capable of running this task. That typically means the task was written for Windows only. For example, written for Windows Desktop PowerShell.
2019-09-12T13:55:21.8361383Z ##[debug]System.Exception: The current operating system is not capable of running this task. That typically means the task was written for Windows only. For example, written for Windows Desktop PowerShell.
at Microsoft.VisualStudio.Services.Agent.Worker.TaskRunner.RunAsync()
at Microsoft.VisualStudio.Services.Agent.Worker.StepsRunner.RunStepAsync(IStep step, CancellationToken jobCancellationToken)
2019-09-12T13:55:21.8365066Z ##[section]Finishing: Terraform plan
The pipelines fails on "terraform plan". Here's the yml:
variables:
env: 'environment'
steps:
- task: petergroenewegen.PeterGroenewegen-Xpirit-Vsts-Release-Terraform.Xpirit-Vsts-Release-Terraform.Terraform#2
displayName: 'Terraform plan'
inputs:
TemplatePath: '$(System.DefaultWorkingDirectory)/_repository/tf'
Arguments: 'plan -var-file=$(System.DefaultWorkingDirectory)/_repository/tf/$(env)/$(env).tfvars '
InstallTerraform: true
UseAzureSub: true
ConnectedServiceNameARM: 'deploy-sco'
ManageState: true
SpecifyStorageAccount: true
StorageAccountResourceGroup: 'rg-terraform'
StorageAccountRM: sta
StorageContainerName: terraform
InitArguments: '-backend-config=$(System.DefaultWorkingDirectory)/_repository/tf/$(env)/$(env).beconf'
The build agent is a docker container, built with the following dockerfile
FROM ubuntu:16.04
ENV DEBIAN_FRONTEND=noninteractive
RUN echo "APT::Get::Assume-Yes \"true\";" > /etc/apt/apt.conf.d/90assumeyes
RUN apt-get update && apt-get install -y --no-install-recommends ca-certificates curl jq git iputils-ping libcurl3 libunwind8 netcat libssl-dev unzip wget apt-utils apt-transport-https make binutils gcc lsb-release gnupg
RUN wget -P /tmp/download https://releases.hashicorp.com/terraform/0.12.7/terraform_0.12.7_linux_amd64.zip
RUN wget -P /tmp/download -q https://packages.microsoft.com/config/ubuntu/16.04/packages-microsoft-prod.deb
RUN unzip /tmp/download/terraform_0.12.7_linux_amd64.zip -d /tmp/download/
RUN mv /tmp/download/terraform /usr/local/bin
RUN chmod a+x /usr/local/bin/terraform
RUN apt-get install /tmp/download/packages-microsoft-prod.deb
RUN apt-get update
RUN apt-get -y install powershell
RUN wget -P /tmp/download -q https://curl.haxx.se/download/curl-7.65.3.tar.gz
RUN cd /tmp/download; tar xzf curl-7.65.3.tar.gz
RUN cd /tmp/download/curl-7.65.3; ./configure --prefix=/opt/curl-7.65.3 --disable-ipv6 --with-ssl; make; make install
RUN mv /usr/bin/curl /tmp; ln -s /opt/curl-7.65.3/bin/curl /usr/bin/curl
RUN curl -sL https://packages.microsoft.com/keys/microsoft.asc | gpg --dearmor | tee /etc/apt/trusted.gpg.d/microsoft.asc.gpg > /dev/null
RUN AZ_REPO=$(lsb_release -cs); echo "deb [arch=amd64] https://packages.microsoft.com/repos/azure-cli/ $AZ_REPO main" | tee /etc/apt/sources.list.d/azure-cli.list
RUN apt-get update
RUN apt-get install azure-cli
RUN pwsh -c "Install-Module -Name Az -Force"
RUN pwsh -c "Install-Module -Name Azure -Force"
WORKDIR /azp
COPY ./start.sh .
RUN chmod +x start.sh
CMD ["./start.sh"]

Answer 1

As what you said, yes, the Terraform plan can be compatible including linux, windows and MacOS. But now, the issue you are facing, is the extension and task you are using which named Run Terraform task can only be executed in the agent which installed on Windows. You can see its doc to know that: Getting started with Terraform on Windows and Azure.
There has another extensions which created by our Microsoft DevLabs and individual developer: Terraform and Terraform Build & Release Tasks. The tasks in these two extensions can all compiled in windows, linux and macOS.
You'd better change to use the tasks which in these two extensions.