provider/docker: add option to keep local image on destroy

[xsellier]

Working with many docker container, I need to keep some local images even if I want to destroy the container, I want to keep the image locally. So I added the option keep_locally.

[stack72]

This LGTM! Tests are green and i can see that the docker image exists after the tests run

TF_LOG=1 make testacc TEST=./builtin/providers/docker TESTARGS='-run=TestAccDockerImage' 2>~/tf.log
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /vendor/)
TF_ACC=1 go test ./builtin/providers/docker -v -run=TestAccDockerImage -timeout 120m
=== RUN   TestAccDockerImage_basic
--- PASS: TestAccDockerImage_basic (42.22s)
=== RUN   TestAccDockerImage_private
--- PASS: TestAccDockerImage_private (167.67s)
=== RUN   TestAccDockerImage_destroy
--- PASS: TestAccDockerImage_destroy (85.13s)
PASS
ok      github.com/hashicorp/terraform/builtin/providers/docker 295.044s

terraform [xsellier-feature/docker-keep-image-on-destroy●] % docker images
REPOSITORY          TAG                 IMAGE ID            CREATED             SIZE
crux                3.1                 7e0975b8569c        6 months ago        341.7 MB

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