add an option in config to pull base image when build dockerfile

[kattunga]

Please,
add a config option to add the --pull parameter when build dockerfile.
Now there is no option to pull and update base images.

Another option could be a second menu entry in context menu with this option so we could have:
Build Image...
Build Image (pull base image)...

[bwateratmsft]

This is a good suggestion, thanks!

[bwateratmsft]

@EricJizbaMSFT @sgreenmsft @fiveisprime @PrashanthCorp @philliphoff

For .NET Core debugging in particular, I think it would make sense for there to be a "pull" argument in the "docker build" section, defaulting to true. This would help the common stale image issues.

For the right click -> Build Image scenario, I think we could do either a configuration option, or have two commands like @kattunga suggests. Any thoughts?

[bwateratmsft]

Resolved in #1245