Fix Git checkout in container

This lets packages resolve Git refs in their source tree. GEANT4 needs this.

alibuild_helpers/git.py

@@ -49,7 +49,11 @@ def cloneReferenceCmd(self, source, referenceRepo, usePartialClone):
   def cloneSourceCmd(self, source, destination, referenceRepo, usePartialClone):
     cmd = ["clone", "-n", source, destination]
     if referenceRepo:
-      cmd.extend(["--reference", referenceRepo])
+      # If we're building inside a Docker container, we can't refer to the
+      # mirror repo directly, since Git uses an absolute path. With
+      # "--dissociate", we still copy the objects locally, but we don't refer
+      # to them by path.
+      cmd.extend(["--dissociate", "--reference", referenceRepo])
     if usePartialClone:
       cmd.extend(clone_speedup_options())
     return cmd

tests/test_build.py

@@ -105,7 +105,7 @@
                            "/sw/MIRROR/zlib", "--filter=blob:none"), ".", False
 GIT_CLONE_SRC_ZLIB_ARGS = ("clone", "-n", "https://github.com/star-externals/zlib",
                            "/sw/SOURCES/zlib/v1.2.3/8822efa61f",
-                           "--reference", "/sw/MIRROR/zlib", "--filter=blob:none"), ".", False
+                           "--dissociate", "--reference", "/sw/MIRROR/zlib", "--filter=blob:none"), ".", False
 GIT_SET_URL_ZLIB_ARGS = ("remote", "set-url", "--push", "origin", "https://github.com/star-externals/zlib"), \
     "/sw/SOURCES/zlib/v1.2.3/8822efa61f", False
 GIT_CHECKOUT_ZLIB_ARGS = ("checkout", "-f", "master"), \
@@ -115,7 +115,7 @@
                            "+refs/heads/*:refs/heads/*"), "/sw/MIRROR/root", False
 GIT_CLONE_SRC_ROOT_ARGS = ("clone", "-n", "https://github.com/root-mirror/root",
                            "/sw/SOURCES/ROOT/v6-08-30/f7b3366117",
-                           "--reference", "/sw/MIRROR/root", "--filter=blob:none"), ".", False
+                           "--dissociate", "--reference", "/sw/MIRROR/root", "--filter=blob:none"), ".", False
 GIT_SET_URL_ROOT_ARGS = ("remote", "set-url", "--push", "origin", "https://github.com/root-mirror/root"), \
     "/sw/SOURCES/ROOT/v6-08-30/f7b3366117", False
 GIT_CHECKOUT_ROOT_ARGS = ("checkout", "-f", "v6-08-00-patches"), \