Builds with a pinned Docker label fail mysteriously

When trying to build the [original code](https://github.com/Automattic/wp-calypso/pull/25321/commits/addb44ad284fceedc103d855369cf85eb94eb8b1) in https://github.com/Automattic/wp-calypso/pull/25321, dserve would bomb with the following log:

```
Time=2018-06-06T20:08:01.651Z | ---------------- DOCKER START ----------------
Time=2018-06-06T20:08:07.378Z | Step 1/16 : FROM node:8.11.2@sha256:321655aeb195b7946e9a29d28453388751389e8ced66b4dea772ae76a6985309
Time=2018-06-06T20:08:07.902Z | { status: 'Pulling from library/node', id: '8.11.2' }
Time=2018-06-06T20:08:08.182Z | Encountered error when building image
```

That PR was trying to pin the docker images to make builds more stable. Currently we're using labels from docker, which can change as the underlying OS takes patches.

Renovate (the new dependency updating tool) takes care of creating new PRs as the docker image associated with the label changes.

It's not critical that this get fixed (we decided to stick with plain labels, they've worked just fine so far), but it might be good to understand what went wrong.

Start by reproducing the Docker build with the pinned FROM node:8.11.2@sha256 example and inspect how dserve reports the failed build. Compare that behavior with a plain-label build; done means identifying why the pinned image fails and fixing or documenting the behavior.

Written by the indexing model from the issue text.