feat(docker): use whole tag as currentValue (#2922)

lib/config/definitions.js

lib/datasource/docker.js

lib/manager/circleci/extract.js

lib/manager/docker-compose/extract.js

lib/manager/dockerfile/extract.js

lib/manager/dockerfile/update.js

lib/manager/kubernetes/extract.js

lib/versioning/docker/index.js

lib/workers/branch/index.js

lib/workers/repository/process/lookup/index.js

lib/workers/repository/updates/branchify.js

test/datasource/docker.spec.js

test/manager/circleci/__snapshots__/extract.spec.js.snap

...manager/docker-compose/__snapshots__/extract.spec.js.snap

test/manager/dockerfile/__snapshots__/extract.spec.js.snap

test/manager/dockerfile/__snapshots__/update.spec.js.snap

test/manager/dockerfile/extract.spec.js

test/manager/dockerfile/update.spec.js

test/manager/gitlabci/__snapshots__/extract.spec.js.snap

test/manager/kubernetes/__snapshots__/extract.spec.js.snap