fix: image credentials for services must not override container image credentials (#181)

- do not override username and password when looping over services
- split prepareJobContainer out of startJobContainer
- split getNetworkName out as it is used by both
- add unit tests for prepareJobContainer
- make containre.NewContainer mockable
- add MockVariable helper

Closes forgejo/runner#575

---

Note to reviewers: do not show whitespace change, the refactor will show in  a minimal way. When the fix is reverted the tests fail as follows:

```
        	            	Diff:
        	            	--- Expected
        	            	+++ Actual
        	            	@@ -81,4 +81,4 @@
        	            	   Image: (string) (len=10) "some:image",
        	            	-  Username: (string) (len=17) "containerusername",
        	            	-  Password: (string) (len=17) "containerpassword",
        	            	+  Username: (string) (len=16) "service2username",
        	            	+  Password: (string) (len=16) "service2password",
        	            	   Entrypoint: ([]string) (len=3) {
        	Test:       	TestStartJobContainer/Overlapping
```

Reviewed-on: https://code.forgejo.org/forgejo/act/pulls/181
Reviewed-by: Michael Kriese <michael.kriese@gmx.de>
Co-authored-by: Earl Warren <contact@earl-warren.org>
Co-committed-by: Earl Warren <contact@earl-warren.org>

act/testutils/mockvariable.go

@@ -0,0 +1,10 @@
+// Copyright 2025 The Forgejo Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package testutils
+
+func MockVariable[T any](p *T, v T) (reset func()) {
+	old := *p
+	*p = v
+	return func() { *p = old }
+}