Add helm env variables explicitly to pulumi Dockerfile

[kahkeng]

If the HOME environment variable is changed (say when the image is used in Github Actions), helm fails to find the stable repo, which can cause Pulumi to be unable to install helm charts.

Addresses #4397.

Here's an example of the problem before the fix.

Running helm in the original image shows the stable repo that has been added:

$ docker run -it --rm --entrypoint=bash pulumi/pulumi
root@e838f313ee27:/# helm repo list
NAME    URL
stable  https://kubernetes-charts.storage.googleapis.com

But overriding the HOME environment variable (to /home/username instead of `/root) causes an issue:

$ docker run -it --rm --entrypoint=bash -e HOME pulumi/pulumi
root@a822180926ba:/# helm repo list
Error: no repositories to show

After rebuilding the docker image with the fix, we get the desired behavior:

$ docker run -it --rm --entrypoint=bash -e HOME kahkeng/pulumi
root@b6896de32972:/# helm repo list
NAME    URL
stable  https://kubernetes-charts.storage.googleapis.com

[lblackstone]

LGTM

[lblackstone]

Thanks for the fix, @kahkeng!