[devcontainer] Added shellenv to dockerfile template

[mohsenari]

Summary

title says it.
fixes #1028

How was it tested?

• devbox init
• devbox add terraform
• devbox generate devcontainer
• add a file in .vscode/tasks.json and put the following json in it:

 {
    "version": "2.0.0",
    "tasks": [
        {
            "label": "terraform version",
            "type": "shell",
            "command": "terraform version"
        }
    ]
}

• cmd + shift + p and select "Rebuild and Reopen in Container"
• cmd + shift + p and select "Run Task"
• Choose terraform task to run
• Assert that the output shows terraform version is printed in output

[Lagoja]

LGTM

[apgrucza]

This line seems redundant

[mohsenari]

@apgrucza true. But the problem is we use this template file for both devcontainer setup and for general docker setup.
I think the best way is to remove the CMD and keep shellenv for devcontainer and vice versa for devbox generate dockerfile.

[apgrucza]

Ok, but maybe the same change would be beneficial for the general Dockerfile too?