Since I upgraded to v1.5.0.beta3 +126 (by rebuilding the Docker image), letter avatars are unaligned:
Note that I’m not using the external avatar service. If I do turn that on, the avatars look different and are aligned:
I’ve cleared browser cache as appropriate and so on. 
I believe @mpalmer did some alignments fixes to our service that might be present in discourse itself. Maybe he can chime in here with what needs to happen to get that fixed?
The problem here is that a range of versions of ImageMagick have a bug in them that causes centered letters to be vertically misaligned. The bug thread on the IM forums says that the bug was fixed in 6.9.2-4 Beta, so it would be good if you could check what version of IM you’ve got (./launcher enter app, then run convert -h |grep Version:). If you’re running a “fixed” version (6.9.2-4 or above) then presumably the bug isn’t fully fixed. If you’re running a buggy version (6.9.1-4 to 6.9.2-3), we’ll need to work out why a fresh rebuild is getting the buggy IM release.
Not our bug, check ImageMagick versions.
There’s a temporary workaround.