unit-test-docker: replace valgrind-dbg

Get the following error with a clean run of this script:
base: Package valgrind-dbg is not available, but is referred to by
      another package.
base: This may mean that the package is missing, has been obsoleted, or
base: is only available from another source
base: However the following packages replace it:
base:   valgrind-mpi-dbgsym valgrind-dbgsym
base:
base: E: Package 'valgrind-dbg' has no installation candidate

It doesn't seem the valgrind-mpi package is needed for our use cases so
just add valgrind-dbgsym

Tested:
- Verified script now runs without error

Change-Id: Id0915c1dfa442dd02425ade14334e19538741067
Signed-off-by: Andrew Geissler <geissonator@yahoo.com>
diff --git a/scripts/build-unit-test-docker b/scripts/build-unit-test-docker
index a444e7f..cbc4bd8 100755
--- a/scripts/build-unit-test-docker
+++ b/scripts/build-unit-test-docker
@@ -830,7 +830,7 @@
     sudo \
     systemd \
     valgrind \
-    valgrind-dbg \
+    valgrind-dbgsym \
     vim \
     wget \
     xxd