Bump 1.21.1

[shin-]

Automated release for docker-compose 1.21.1

Bugfixes

• In 1.21.0, we introduced a change to how project names are sanitized for
  internal use in resource names. This caused issues when manipulating an
  existing, deployed application whose name had changed as a result.
  This release properly detects resources using "legacy" naming conventions.

• Fixed an issue where specifying an in-context Dockerfile using an absolute
  path would fail despite being valid.

• Fixed a bug where IPAM option changes were incorrectly detected, preventing
  redeployments.

• Validation of v2 files now properly checks the structure of IPAM configs.

• Improved support for credentials stores on Windows to include binaries using
  extensions other than .exe. The list of valid extensions is determined by
  the contents of the PATHEXT environment variable.

• Fixed a bug where Compose would generate invalid binds containing duplicate
  elements with some v3.2 files, triggering errors at the Engine level during
  deployment.

[GordonTheTurtle]

Please sign your commits following these rules:
https://github.com/moby/moby/blob/master/CONTRIBUTING.md#sign-your-work
The easiest way to do this is to amend the last commit:

$ git clone -b "bump-1.21.1" [email protected]:docker/compose.git somewhere
$ cd somewhere
$ git rebase -i HEAD~842354462536
editor opens
change each 'pick' to 'edit'
save the file and quit
$ git commit --amend -s --no-edit
$ git rebase --continue # and repeat the amend for each commit
$ git push -f

Amending updates the existing PR. You DO NOT need to open a new one.