Update docker

Changes:

- https://github.com/docker-library/docker/commit/1b50c31: Update to 19.03.0-beta3

library/docker

@@ -4,17 +4,17 @@ Maintainers: Tianon Gravi <[email protected]> (@tianon),
              Joseph Ferguson <[email protected]> (@yosifkit)
 GitRepo: https://github.com/docker-library/docker.git
 
-Tags: 19.03.0-beta2, 19.03-rc, rc, test
+Tags: 19.03.0-beta3, 19.03-rc, rc, test
 Architectures: amd64, arm32v6, arm64v8, ppc64le, s390x
-GitCommit: a923d44d0fe9b53f73c979fef5206bac0f5f37e7
+GitCommit: 1b50c312fcc657c152da1d6d72c4b554055c729a
 Directory: 19.03-rc
 
-Tags: 19.03.0-beta2-dind, 19.03-rc-dind, rc-dind, test-dind
+Tags: 19.03.0-beta3-dind, 19.03-rc-dind, rc-dind, test-dind
 Architectures: amd64, arm32v6, arm64v8, ppc64le, s390x
 GitCommit: 27471a8b93e980bd4c51464ee933ed90fd36bf97
 Directory: 19.03-rc/dind
 
-Tags: 19.03.0-beta2-git, 19.03-rc-git, rc-git, test-git
+Tags: 19.03.0-beta3-git, 19.03-rc-git, rc-git, test-git
 Architectures: amd64, arm32v6, arm64v8, ppc64le, s390x
 GitCommit: 98ffef81ebfa7601a9ed2f0bf56d78f426bf253c
 Directory: 19.03-rc/git