Fix comment on workdir (very confusing)

[ekzhang]

Extremely confusing field called workdir on ImageMetadata which is not actually used by any of our systems, ImageManifest.workdir is the actual workdir. This is wrong in 99% of images.

[mwaskom]

Not sure I know the initial intention here or exactly the nature of the confusion, but I think this is meant to convey what the workdir state would be for subsequent builder tasks, (i.e. so that we can know where .add_local_* layers would put their files with a relative remote location. @freider might know more.

[ekzhang]

Yeah there's a somewhat confusing issue in that image.manifest.workdir and image.workdir are different. image.manifest.workdir is the actual workdir, and it's read by tasks on startup:

• runner tasks: uses manifest.workdir, otherwise /root if not set
• builder tasks: uses manifest.workdir, otherwise / if not set
• sandbox tasks: uses sandbox.workdir, otherwise manifest.workdir if not set, otherwise / if not set

Meanwhile image.workdir is not read by anything in the runtime and is a duplicate metadata value that is measured by builder.

The issue is that for the vast majority of tasks, manifest.workdir is not set, but in the internal lookup and database for images, we see workdir: / observed by this metadata process. Which is misleading — it's actually not set, and the runtime code treats it as not set.