Refs #11623

Added capabilities block
Added tests for it
Added documentation for it.

My PC doesnt support memory swap so it errors there.

```
$ make testacc TEST=./builtin/providers/docker TESTARGS='-run=TestAccDockerContainer_'
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /terraform/vendor/)
2017/02/17 14:57:08 Generated command/internal_plugin_list.go
TF_ACC=1 go test ./builtin/providers/docker -v -run=TestAccDockerContainer_ -timeout 120m
=== RUN   TestAccDockerContainer_basic
--- PASS: TestAccDockerContainer_basic (44.50s)
=== RUN   TestAccDockerContainer_volume
--- PASS: TestAccDockerContainer_volume (40.73s)
=== RUN   TestAccDockerContainer_customized
--- FAIL: TestAccDockerContainer_customized (50.27s)
	testing.go:265: Step 0 error: Check failed: Check 2/2 error: Container has wrong memory swap setting: -1
	Please check that you machine supports memory swap (you can do that by running 'docker info' command).
=== RUN   TestAccDockerContainer_upload
--- PASS: TestAccDockerContainer_upload (38.56s)
FAIL
exit status 1
FAIL	github.com/hashicorp/terraform/builtin/providers/docker	174.070s
Makefile:48: recipe for target 'testacc' failed
make: *** [testacc] Error 1
```