cli/command/image: trim iidfile image ID and fail clearly when missing

[HarnageaGabriel]

• What I did

When docker build --iidfile <file> is used, runBuild picks up the
resulting image ID from the daemon's aux message or, in quiet mode,
from the build output, and writes it to the file as-is.

That has two rough edges:

• the quiet-mode value can carry surrounding whitespace / a trailing
  newline, which then lands in the iidfile and is misread by tools that
  consume it;
• an all-whitespace value slipped past the old imageID == "" check and
  was written out as an effectively empty file.

This trims the value before use and treats an empty result as "no image
ID", returning a clear error rather than writing a broken file.

• Related issue

Addresses the failure mode in #2971 ("cannot write /tmp/... because
server did not provide an image ID"), where parallel Compose builds
sharing an iidfile — or older daemons — could leave the CLI without an
ID. The CLI can't invent an ID it wasn't given, but it now fails
predictably and never leaves a bogus iidfile behind (the file is already
pre-removed before the build).

• How to test

go test ./cli/command/image/ -run TestRunBuild

New tests:

• TestRunBuildWithIidFileWritesImageID — an aux ID is written to the
  iidfile with no surrounding whitespace;

• TestRunBuildWithoutImageIDReturnsError — no ID from the daemon yields
  an error and no iidfile is created.

• Description for the changelog

Fix docker build --iidfile writing a stray newline into the image ID
file, and give a clear error when the daemon returns no image ID.