Propagate context.Context to build logic

[michaeldwan]

I'm doing some work in the build code that requires a context.Context (adding another command.Command implementation that hits the docker engine directly). Since most of the build code doesn't accept a context, I had to make a ton of tiny changes to pass a context from a command's run function through to the dockerfile generators and the docker command.Command interface.

These changes shouldn't have a meaningful impact, but extracting them into a separate PR will make the next one much easier to review. 😅

The vast majority of changes are adding ctx context.Context to function signatures then passing it on. The only actual changes that use the context are:

• calls to the docker CLI that used exec.Command(... now use exec.CommandContext(ctx, ..., such as this example.
• several function calls that required a context were using context.Background() or context.TODO() and are now using the parent context provided (such as this)