Add ReadOnlyRootFilesystem flag

Adds flag to denote root-filesystem should be made read-only.

Needed for downstream work and re-vendoring in faas-swarm/faas-
netes and openfaas-operator.

Closes #723

Signed-off-by: Alex Ellis (VMware) <alexellis2@gmail.com>

gateway/requests/requests.go

@@ -41,6 +41,10 @@ type CreateFunctionRequest struct {
 
 	// Requests of resources requested by function
 	Requests *FunctionResources `json:"requests"`
+
+	// ReadOnlyRootFilesystem removes write-access from the root filesystem
+	// mount-point.
+	ReadOnlyRootFilesystem bool `json:"readOnlyRootFilesystem"`
 }
 
 // FunctionResources Memory and CPU