Build shim 15.4 from tarball

Removing ability to build shim from a tag - we're going to assume that
tarballs are the preferred method of building from this point on.

Dockerfile

diff --git a/Dockerfile b/Dockerfile
index 9d35b63..73312ab 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -9,22 +9,13 @@
     patch \
     wget
 
-# pnardini: We need to build shim 15.4 from a tarball.  Download and extract it.
+# pnardini: We need to build shim 15.4 from a tarball now.  Download and extract it.
 RUN mkdir -p /build/shim
 WORKDIR /build/shim
 RUN wget https://github.com/rhboot/shim/releases/download/15.4/shim-15.4.tar.bz2
 RUN tar -jxvpf shim-15.4.tar.bz2 && rm shim-15.4.tar.bz2
 WORKDIR /build/shim/shim-15.4
 
-# pnardini: The below section has been commented out for shim 15.4.  Not sure if
-# this is a one-off or if tarballs will be how we build shim going forward, so
-# leaving this section here for now.
-
-# Clone shim, check out tag 15.4, and init submodules
-# RUN git clone --branch 15.4 https://github.com/rhboot/shim.git /build/shim
-# WORKDIR /build/shim
-# RUN git submodule update --init
-
 # Add our public certificate
 ADD neverware.cer .