Update .dockerignore to ensure glyphs are included

[why]
This fixes the Docker image build after changes in 9633b4d moved
PowerlineSymbols.otf into a subdirectory. Previously, the
.dockerignore line for src/glyphs/*.* would exclude any
subdirectories in src/glyphs from being included. Without this
change, a complete font build fails.

.dockerignore

@@ -1,4 +1,4 @@
 **
-!src/glyphs/*.*
+!src/glyphs
 !font-patcher
 !bin/scripts/docker-entrypoint.sh