build-unit-test-docker: upgrade to ubuntu plucky

GCC-15 does not exist in previous ubuntu versions.  Update to plucky
so that we can get GCC-15 pulled in, which is what is used on the
latest Yocto.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I7c9e7b8bae9dc81ffa2b868d2921bb6f972d5ecd
diff --git a/scripts/build-unit-test-docker b/scripts/build-unit-test-docker
index fc91603..6ad03dd 100755
--- a/scripts/build-unit-test-docker
+++ b/scripts/build-unit-test-docker
@@ -715,7 +715,7 @@
 )
 force_build = os.environ.get("FORCE_DOCKER_BUILD")
 is_automated_ci_build = os.environ.get("BUILD_URL", False)
-distro = os.environ.get("DISTRO", "ubuntu:oracular")
+distro = os.environ.get("DISTRO", "ubuntu:plucky")
 branch = os.environ.get("BRANCH", "master")
 ubuntu_mirror = os.environ.get("UBUNTU_MIRROR")
 docker_reg = os.environ.get("DOCKER_REG", "public.ecr.aws/ubuntu")