add -lz to the libraries linked against by mkmf's TRY_LINK

which wasn't necessary on manylinux2014 but seems to be necessary on
ubuntu:20.04 (but just for intel+gnu? weird.) to resolve this error
building native extensions:

  LD_LIBRARY_PATH=.:/usr/local/rake-compiler/ruby/x86_64-linux-gnu/ruby-3.3.0-rc1/lib
  "x86_64-linux-gnu-gcc -o conftest
  -I/usr/local/rake-compiler/ruby/x86_64-linux-gnu/ruby-3.3.0-rc1/include/ruby-3.3.0+0/x86_64-linux-gnu
  -I/usr/local/rake-compiler/ruby/x86_64-linux-gnu/ruby-3.3.0-rc1/include/ruby-3.3.0+0/ruby/backward
  -I/usr/local/rake-compiler/ruby/x86_64-linux-gnu/ruby-3.3.0-rc1/include/ruby-3.3.0+0
  -I../../../../ext/mri -O1 -fno-omit-frame-pointer -fno-fast-math
  -fstack-protector-strong -s -fPIC conftest.c
  -L. -L/usr/local/rake-compiler/ruby/x86_64-linux-gnu/ruby-3.3.0-rc1/lib
  -Wl,-rpath,/usr/local/rake-compiler/ruby/x86_64-linux-gnu/ruby-3.3.0-rc1/lib
  -L. -pipe -s -fstack-protector-strong -rdynamic -Wl,-export-dynamic
  -Wl,--no-as-needed -lruby-static -lpthread -lrt -ldl -lcrypt
  -Wl,-rpath,/usr/local/rake-compiler/ruby/x86_64-linux-gnu/ruby-3.3.0-rc1/lib
  -lruby -lm -lpthread -lc"

  /usr/bin/ld: /usr/local/rake-compiler/ruby/x86_64-linux-gnu/ruby-3.3.0-rc1/lib/libruby-static.a(addr2line.o): in function `fill_lines':
  addr2line.c:(.text+0x1bc0): undefined reference to `uncompress'
  collect2: error: ld returned 1 exit status

Author: flavorjones

Dockerfile.mri.erb

@@ -147,7 +147,7 @@ RUN bash -c " \
 RUN find /usr/local/rake-compiler/ruby/*linux*/ -name libruby.so | xargs rm
 RUN find /usr/local/rake-compiler/ruby/*linux*/ -name libruby-static.a | while read f ; do cp $f `echo $f | sed s/-static//` ; done
 RUN find /usr/local/rake-compiler/ruby/*linux*/ -name libruby.a | while read f ; do ar t $f | xargs ar d $f ; done
-RUN find /usr/local/rake-compiler/ruby/*linux*/ -name mkmf.rb | while read f ; do sed -i ':a;N;$!ba;s/TRY_LINK = [^\n]*\n[^\n]*\n[^\n]*LOCAL_LIBS)/& -lruby-static -lpthread -lrt -ldl <% if platform =~ /x86/ %> -lcrypt <% end %>/' $f ; done
+RUN find /usr/local/rake-compiler/ruby/*linux*/ -name mkmf.rb | while read f ; do sed -i ':a;N;$!ba;s/TRY_LINK = [^\n]*\n[^\n]*\n[^\n]*LOCAL_LIBS)/& -lruby-static -lpthread -lrt -ldl <% if platform =~ /x86/ %> -lcrypt <% if platform !~ /musl/ %> -lz <% end %> <% end %>/' $f ; done
 <% end %>
 
 <% if platform =~ /mingw/ %>