Dockerfile.saithrift-client has a static tag in the FROM instruction

[aputriax]

The idea is that we are pulling the latest image of saithrift-client-bldr to build local saithrift-client container.
So when there are any updates in Dockerfile.saithrift-client-bldr, its md5 is changed and a new image is built - and saithrift-client has to use it.
But, since Dockerfile.saithrift-client has a static tag pointing to the 220819 tag, the new bldr container won't be used.

[KrisNey-MSFT]

Not a bug, paves the way for self-maintaining build environment (nice to have)