Modify dockerfile

[moriyama]

I fixed the following two reviewer comments in the Dockerfile:

rhboot#421 (comment)
steve-mcintyre wrote on Sep 9, 2024:

• I don't see why you're grabbing Peter's
  shim-unsigned-x64-15.8-2.el9.src.rpm rather than just using the
  upstream shim tarball directly. This isn't a blocker for this review
  as it's possible to check the tarball checksum from the
  src.rpm. Please don't do this again, though.

rhboot#421 (comment)
aronowski wrote on Dec 23, 2024:

Prefer using older minor system releases - the shim binary build does reproduce correctly now, but in case new tooling gets shipped as part of EL9.6, that can change unexpectedly.

[moriyama]

I added check that the checksum of the downloaded shim tarball is correct.