provider/docker: don't crash with empty commands

If any of the entries in `commands` on `docker_container` resources was
empty, the assertion to string panic'd. Since we can't use ValidateFunc
on list elements, we can only really check this at apply time. If any
value is nil (resolves to empty string during conversion), we fail with
an error prior to creating the container.

Fixes #6409.

builtin/providers/docker/resource_docker_container_funcs.go

@@ -51,6 +51,11 @@ func resourceDockerContainerCreate(d *schema.ResourceData, meta interface{}) err
 
 	if v, ok := d.GetOk("command"); ok {
 		createOpts.Config.Cmd = stringListToStringSlice(v.([]interface{}))
+		for _, v := range createOpts.Config.Cmd {
+			if v == "" {
+				return fmt.Errorf("values for command may not be empty")
+			}
+		}
 	}
 
 	if v, ok := d.GetOk("entrypoint"); ok {
@@ -269,6 +274,10 @@ func resourceDockerContainerDelete(d *schema.ResourceData, meta interface{}) err
 func stringListToStringSlice(stringList []interface{}) []string {
 	ret := []string{}
 	for _, v := range stringList {
+		if v == nil {
+			ret = append(ret, "")
+			continue
+		}
 		ret = append(ret, v.(string))
 	}
 	return ret