Ensure local images are up to date

[dbreshears]

[bwateratmsft]

We have the dockerBuild.pull flag today on docker-build tasks; if true it is equivalent to using --pull. Currently it is treated as false if unspecified, but we can change that.

[philliphoff]

We should continue to treat unspecified as false, but we should instead scaffold pull to true in new tasks.

[eine]

We should continue to treat unspecified as false, but we should instead scaffold pull to true in new tasks.

Since the default behaviour in docker is false, I think it should be better kept like that.

[philliphoff]

Since the default behaviour in docker is false, I think it should be better kept like that.

Thanks for the feedback. We've had discussions related to this behavior in Visual Studio, too, but users there seem to prefer the images kept up to date despite the Docker CLI's defaults, hence our wanting to copy that behavior in VS Code as well. Scaffolding pull to true for new tasks is a bit of a compromise over simply changing the interpretation of omitting the value on a task; it also allows for changing our minds if VS Code users end up preferring the opposite behavior.