How do I specify a PowerShell script as a Docker container entry point?

Question

I need to run a [Windows] Docker container as an executable, running a rather involved PowerShell script (invoking Java and .NET applications) and exiting. Docker documentation suggests using ENTRYPOINT for this purpose. So I went ahead and created a Dockerfile with the following contents:

FROM microsoft/dotnet-framework
COPY run.ps1 /
ENTRYPOINT [ "powershell.exe", "C:\\run.ps1" ]

The contents of run.ps1 (uber-simplified for this question):

gci
write-host "looks like everything is good!"

Then, I ran the following commands:

# Build the Docker image
docker build --rm -t testdockerps .

# Create/run container using above image
docker run -it testdockerps

The container ran successfully, displaying the contents of C:\ followed by the message - looks like everything is good!.

I have a couple of questions based on what my observations:

1. What is the default shell for a Windows based Docker container? Is there any way to set it to PowerShell, so I don't have to specify "powershell" as the first element of the ENTRYPOINT JSON array? Should I be using the SHELL command in the Dockerfile?
2. Creating and running the container takes about 3-4 seconds which is somewhat understandable, but after the PS1 script completes, it takes nearly a questionable 10 seconds for the container to exit and return to the command prompt. What may be the cause of this delay?

Trihedral

Answer 1

Yes you can specify powershell as default shell like below on top of DOCKERFILE

SHELL ["powershell", "-Command", "$ErrorActionPreference = 'Stop'; $ProgressPreference = 'Continue'; $verbosePreference='Continue';"]

I'm not sure you can do anything about the time it takes to spin down your VM

Disinherit

Adding SHELL did not take any effect, i.e. I still needed to specify "powershell" as the first argument to ENTRYPOINT. Otherwise, the Docker container fails. It looks like SHELL is only useful for RUN commands defined in the Dockerfile, but it does not actually set PowerShell as the default shell in the container. I learnt something too, that I am using the exec form of ENTRYPOINT, so there is apparently no shell. But still, how does the container know which type of runtime environment to pick up if specify Write-Host "hello world" versus echo hello world? – Trihedral

Below works for me FROM microsoft/windowsservercore:1709 SHELL ["powershell", "-Command", "$ErrorActionPreference = 'Stop'; $ProgressPreference = 'Continue'; $verbosePreference='Continue';"] ENTRYPOINT Write-output "hello" – Disinherit

Thanks for that clarification. I wanted to set an environment variable in the container and override the entry point to print all the environment variables in the container environment. I used this command but am having trouble with it: docker run --entrypoint "powershell" <image> --env HOSTNAME=$Env:COMPUTERNAME "gci Env:*". I get an error with unexpected token and missing expression. But the same command works perfectly fine without the --env parameter. Why is that? – Trihedral

This works just fine for me and print test as output docker run --entrypoint "powershell" --env HOSTNAME="test" microsoft/windowsservercore:1709 "gci Env:HOSTNAME" Try putting you image after --env parameter – Disinherit

Yes, placing the image name after --env worked. Thanks! – Trihedral