Dockerfile: Add bison and flex to Dockerfile for successful binutils build

[sepen]

First of all, congratulations on this great project! It’s really impressive to see ELKS in action.

While running the Docker build command, I encountered two errors related to missing packages required to build binutils:

make[3]: Entering directory '/elks/cross/build/binutils-build/binutils'
  YACC     arparse.c
/elks/cross/build/binutils-src/missing: 81: bison: not found
WARNING: 'bison' is missing on your system.
         You should only need it if you modified a '.y' file.
         You may want to install the GNU Bison package:
         <http://www.gnu.org/software/bison/>

/elks/cross/build/binutils-src/missing: 81: flex: not found
WARNING: 'flex' is missing on your system.
         You should only need it if you modified a '.l' file.
         You may want to install the Fast Lexical Analyzer package:
         <http://flex.sourceforge.net/>

These errors were solved simply by adding bison and flex to the list of dependencies in the Dockerfile

[ghaerr]

Thanks for using ELKS @sepen!