Add workdir option

[tuler]

Adds a workdir option.

Fixes #65

[crazy-max]

I'm ok with this new input while waiting for better support upstream in docker/buildx#1028.

See also my comment and ci failure.

PTAL @tonistiigi

[crazy-max]

Check CI: https://github.com/docker/bake-action/runs/7283346396?check_suite_focus=true#step:3:311

#29 [build-validate build-validate 1/1] RUN --mount=type=bind,target=.,rw <<EOT (set -e...)
#29 0.399 ERROR: Build result differs. Please build first with "docker buildx bake build"
#29 0.402  M dist/index.js
#29 0.402  M dist/index.js.map
#29 ERROR: executor failed running [/bin/sh -c set -e
git add -A
cp -rf /out/* .
if [ -n "$(git status --porcelain -- dist)" ]; then
  echo >&2 'ERROR: Build result differs. Please build first with "docker buildx bake build"'
  git status --porcelain -- dist
  exit 1
fi
]: exit code: 1

[tuler]

Check CI: https://github.com/docker/bake-action/runs/7283346396?check_suite_focus=true#step:3:311

Sorry, I was running yarn all instead.

[crazy-max]

LGTM thanks, PTAL @tonistiigi