You're confusing some things...
The mk file extension is used for makefile extensions.
The stepseq.lv2 source code has a 'git2lv2.mk', but it's used internally for building rules of its own.
As you noticed it does not have any buildroot stuff.
Buildroot files are also *.mk, but with a different target usage.
The stepseq.lv2 project was initially created for MOD, but later on the author added a regular desktop interface as well.
For you, the fast way to modify the stepseq.lv2 project is:
1. copy/clone the code of it locally, somewhere that docker can access
2. adjust the plugins/package/x42-stepseq/x42-stepseq.mk file to build from that folder (ie, local build)
3. remove the custom rules for building variants of x42-stepseq, just do the normal build (which is 8x8 grid)
4. build with "./build x42-stepseq" using mod-plugin-builder
The x42-stepseq project might not be the best one to start learning though.
Because it has a desktop interface, it includes cairo, opengl and other code that is never used on MOD.
It also has some automated rules to be able to build any grid size.
As always, for the initial test and development, I recommend to do it natively, running on your own audio card locally.
You can get mod-host and mod-ui running on your own pc to simulate a MOD, or alternatively use a regular desktop plugin host similar to mod-host/ui, like Carla or Ingen.
Having issues right at the start (like not being able to build things) can be a motivation killer.
So better to start slow to test native at first, I think.