Remove the default entrypoint

[jkodroff]

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Issue details

We have a default entrypoint that calls pulumi, but in my limited experience with the Docker images, I am usually running other commands in the context of a CI/CD pipeline. (i.e. I am using pulumi/pulumi as the image in which I am running CI/CD.) Therefore, I find the default entrypoint not-especially-useful at best, and confusing at worst (because I didn't really expect the entrypoint to be set).

I think we should consider removing it in a future version. I know this would be a breaking change, so we'd likely need to wait until v4 of Pulumi itself as our container versions are aligned with the actual version of the Pulumi CLI. I would also not be heartbroken if we declined to make this change, since the downside of a breaking change may not be worth the upside of a more intuitive (IMO) interface to the container.

Affected area/feature

[im7mortal]

As I understand it, the image is designed for tools like Google Cloud Build or scripts where arguments are passed directly.

For custom images, you can easily override the default entrypoint as follows:

ARG PULUMI_VERSION="latest"
FROM registry.hub.docker.com/pulumi/pulumi:$PULUMI_VERSION

# Override Pulumi's default entrypoint
ENTRYPOINT []

In the terminal, you can override the entrypoint using the --entrypoint flag:

docker run --entrypoint /bin/bash pulumi/pulumi

For example, in Google Cloud Build, you can use the entrypoint field:

steps:
- name: pulumi/pulumi:latest
  entrypoint: "/bin/bash"
  args: ["-c", "echo hello"]

I assume other build tools also have similar ways to override the entrypoint.