remove uses of pkg/stringid.GenerateRandomID()

This utility was only used for testing, and to generate a random
suffix for Dockerfiles. As we don't need the same contract as
pkg/stringid.GenerateRandomID() (not allow all-numeric IDs as they
would not be usable for hostnames), we can use a local test-utility,
and local implementation for the random suffix instead.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

internal/test/randomid.go

@@ -0,0 +1,15 @@
+package test
+
+import (
+	"crypto/rand"
+	"encoding/hex"
+)
+
+// RandomID returns a unique, 64-character ID consisting of a-z, 0-9.
+func RandomID() string {
+	b := make([]byte, 32)
+	if _, err := rand.Read(b); err != nil {
+		panic(err) // This shouldn't happen
+	}
+	return hex.EncodeToString(b)
+}