Added dockerfile key support

Fixes #486
This commit will add `dockerfilepath` key under Dockerstratergy in buildconfig.
dockerfilepath allow us to use dockerfiles which are named different than `Dockerfile` which are placed in context directory.

for example, for a docker-compose file:

```
version: "2"
services:
    foo:
        build:
            context: "./build"
            dockerfile: "Dockerfile-alternate"
        command: "sleep 3600"
```
Resulting buildconfig will be:

```
apiVersion: v1
kind: BuildConfig
...
...
  strategy:
    dockerStrategy:
      dockerfilePath: Dockerfile-alternate
    type: Docker
...
...
```

pkg/loader/compose/compose.go

@@ -76,7 +76,6 @@ func checkUnsupportedKey(composeProject *project.Project) []string {
 		"Uts":           false,
 		"ReadOnly":      false,
 		"Ulimits":       false,
-		"Dockerfile":    false,
 		"Net":           false,
 		"Sysctls":       false,
 		"Networks":      false, // there are special checks for Network in checkUnsupportedKey function
@@ -321,6 +320,7 @@ func (c *Compose) LoadFile(files []string) (kobject.KomposeObject, error) {
 		serviceConfig.ContainerName = composeServiceConfig.ContainerName
 		serviceConfig.Command = composeServiceConfig.Entrypoint
 		serviceConfig.Args = composeServiceConfig.Command
+		serviceConfig.Dockerfile = composeServiceConfig.Build.Dockerfile
 
 		envs := loadEnvVars(composeServiceConfig.Environment)
 		serviceConfig.Environment = envs