feat: add Dockerfile for development environment

- Debian 13 (trixie) based with sdcc 4.5.0 from apt
- Includes gcc, make, xxd, python3, libjson-c-dev, golang-go
- Add .dockerignore to exclude build artifacts
- Add Docker usage section to README (collapsible)

.dockerignore

@@ -0,0 +1,19 @@
+.git/
+.gitignore
+.gitattributes
+.github/
+output/
+installer/output/
+*.bin
+html_data.c
+html_data.h
+*.o
+*.rel
+*.lst
+*.sym
+*.asm
+*.ihx
+*.img
+*.map
+*.mem
+*.lk