Description
Recent 202012 builds from Jenkins are coming out like this:

admin@sonic~$ show ver
SONiC Software Version: SONiC.202012.8-dirty-20210113.013138
Distribution: Debian 10.7
Kernel: 4.19.0-9-2-amd64
Build commit: 063a4859
Build date: Wed Jan 13 08:20:46 UTC 2021
Built by: johnar@jenkins-worker-7

Steps to reproduce the issue:

1. Build an image based on a specific tag or commit hash, w/o any outstanding/uncommitted changes.
2. Check the IMAGE_VERSION in the build output
3. Check show ver on a DUT

Describe the results you received:
Image says "dirty" and has a time stamp like so:

admin@sonic~$ show ver
SONiC Software Version: SONiC.202012.8-dirty-20210113.013138
Distribution: Debian 10.7
Kernel: 4.19.0-9-2-amd64
Build commit: 063a4859
Build date: Wed Jan 13 08:20:46 UTC 2021
Built by: johnar@jenkins-worker-7

Describe the results you expected:
Image version should just have the tag/commit hash that it was built off of, since there are no outstanding changes when we build on Jenkins.

admin@sonic~$ show ver
SONiC Software Version: 202012.7-264ecb18
Distribution: Debian 10.7
Kernel: 4.19.0-9-2-amd64
Build commit: 063a4859
Build date: Wed Jan 13 08:20:46 UTC 2021
Built by: johnar@jenkins-worker-7

Additional information you deem important (e.g. issue happens only occasionally):
If you look at the output in the build logs, it looks like the initial container/buster container has the correct image version, but subsequent ones do not.