Fixed incorrect docker mount arguments generation

The previous implementation combined --mount and -v notation
of argument [1] adding :ro to make the read-only mount point.
E.g. the command [2] called docker with the following
params: [3]. As a result instead of the read-only
folder /tmp/source, the read-write folder /tmp/source/:ro/'
is created.

This PR:
1. substitutes ':ro' with correct ',readonly'.
2. changes 'src=' and 'dst=' with 'source=' and 'target=' as
   it is stated in the documentation [1]
3. introduces the ability to EXPLICITLY create a mountpoint
   with read-write access. To do so it's necessary to add
   ',rw=true' to the --mount argument
4. corrects UTs adds some additional coverage for added
   functionality

[1]
https://docs.docker.com/storage/bind-mounts/#use-a-read-only-bind-mount

[2]
kustomize fn run ./d --mount type=bind,src=$(pwd)/test/,dst=/tmp/source/

[3]
--mount type=bind,src=/home/ubuntu/kpt-functions-catalog/functions/ts/test/,dst=/tmp/source/:ro

kyaml/fn/runtime/runtimeutil/runtimeutil_test.go

@@ -1379,3 +1379,44 @@ metadata:
 		assert.Equal(t, tc.required, fn.Container.Network.Required)
 	}
 }
+
+func Test_StringToStorageMount(t *testing.T) {
+	tests := []struct {
+		in          string
+		expectedOut string
+	}{
+		{
+			in:          "type=bind,src=/tmp/test/,dst=/tmp/source/",
+			expectedOut: "type=bind,source=/tmp/test/,target=/tmp/source/,readonly",
+		},
+		{
+			in:          "type=bind,src=/tmp/test/,dst=/tmp/source/,rw=true",
+			expectedOut: "type=bind,source=/tmp/test/,target=/tmp/source/",
+		},
+		{
+			in:          "type=bind,src=/tmp/test/,dst=/tmp/source/,rw=false",
+			expectedOut: "type=bind,source=/tmp/test/,target=/tmp/source/,readonly",
+		},
+		{
+			in:          "type=bind,src=/tmp/test/,dst=/tmp/source/,rw=",
+			expectedOut: "type=bind,source=/tmp/test/,target=/tmp/source/,readonly",
+		},
+		{
+			in:          "type=tmpfs,src=/tmp/test/,dst=/tmp/source/,rw=invalid",
+			expectedOut: "type=tmpfs,source=/tmp/test/,target=/tmp/source/,readonly",
+		},
+		{
+			in:          "type=tmpfs,src=/tmp/test/,dst=/tmp/source/,rwe=invalid",
+			expectedOut: "type=tmpfs,source=/tmp/test/,target=/tmp/source/,readonly",
+		},
+		{
+			in:          "type=tmpfs,src=/tmp/test/,dst",
+			expectedOut: "type=tmpfs,source=/tmp/test/,target=,readonly",
+		},
+	}
+
+	for _, tc := range tests {
+		s := StringToStorageMount(tc.in)
+		assert.Equal(t, tc.expectedOut, (&s).String())
+	}
+}