fix: getUIDandGID is able to resolve non-existing users and groups (#2106)

* fix: getUIDandGID is able to resolve non-existing users and groups

A common pattern in dockerfiles is to provide a plain uid and gid number, which doesn't neccesarily exist inside the os.

Signed-off-by: Höhl, Lukas <lukas.hoehl@accso.de>

* test: add chown dockerfile

Signed-off-by: Höhl, Lukas <lukas.hoehl@accso.de>

* chore: format

Signed-off-by: Höhl, Lukas <lukas.hoehl@accso.de>

* chore: add comment

Signed-off-by: Höhl, Lukas <lukas.hoehl@accso.de>

* tests: fix chown dockerfile

Signed-off-by: Höhl, Lukas <lukas.hoehl@accso.de>

* refactor: split up getIdsFromUsernameAndGroup func

Signed-off-by: Höhl, Lukas <lukas.hoehl@accso.de>

* fix: implement raw uid logic for LookupUser

Signed-off-by: Höhl, Lukas <lukas.hoehl@accso.de>

* test: add dockerfiles for integration test

* fix: lookup user error message

* test: add dockerfiles for non-existing user testcase

* fix: forgot error check

* tests: fix syscall credentials test

* chore: add debug output for copy command

* tests: set specific gid for integration dockerfile

* tests: fix syscall credentials test

github runner had the exact uid that i was testing on, so the groups were not empty

Signed-off-by: Höhl, Lukas <lukas.hoehl@accso.de>

* tests: fix test script

Signed-off-by: Höhl, Lukas <lukas.hoehl@accso.de>

* chore: apply golangci lint checks

Signed-off-by: Höhl, Lukas <lukas.hoehl@accso.de>

* fix: reset file ownership in createFile if not root owned

Signed-off-by: Höhl, Lukas <lukas.hoehl@accso.de>

* chore: logrus.Debugf missed format variable

Signed-off-by: Höhl, Lukas <lukas.hoehl@accso.de>

* chore(test-script): remove go html coverage

Signed-off-by: Höhl, Lukas <lukas.hoehl@accso.de>

* test(k8s): increase wait timeout

Signed-off-by: Höhl, Lukas <lukas.hoehl@accso.de>

pkg/util/groupids_fallback.go

@@ -45,6 +45,11 @@ type group struct {
 func groupIDs(u *user.User) ([]string, error) {
 	logrus.Infof("Performing slow lookup of group ids for %s", u.Username)
 
+	// user can have no gid if it's a non existing user
+	if u.Gid == "" {
+		return []string{}, nil
+	}
+
 	f, err := os.Open(groupFile)
 	if err != nil {
 		return nil, errors.Wrap(err, "open")