feat: dockerfile and github based CI build for Africastalking variant (#174)

## Summary

* fixed missing error handler in main
* add injectable build string in main
* add (dynamically linked) docker build
* add github CI workflow
* add extra but useful dev files in dev folder

* closes #93

## Notes

* We'll move to a self-hosted CI runner once it is installed on Gitea.

Reviewed-on: urdt/ussd#174
Co-authored-by: Mohammed Sohail <sohailsameja@gmail.com>
Co-committed-by: Mohammed Sohail <sohailsameja@gmail.com>

dev/dialoguss/sample_user.yaml

@@ -0,0 +1,3 @@
+url: http://localhost:7123
+dial: "*384*96#"
+phoneNumber: +254722123456

dev/docker-compose.yaml

@@ -0,0 +1,21 @@
+services:
+  ussd-pg-store:
+    image: postgres:17-alpine
+    restart: unless-stopped
+    user: postgres
+    environment:
+      - POSTGRES_PASSWORD=postgres
+      - POSTGRES_USER=postgres
+    volumes:
+      - ./init_db.sql:/docker-entrypoint-initdb.d/init_db.sql
+      - ussd-pg:/var/lib/postgresql/data
+    ports:
+      - "127.0.0.1:5432:5432"
+    healthcheck:
+      test: ["CMD-SHELL", "pg_isready"]
+      interval: 10s
+      timeout: 5s
+      retries: 5
+volumes:
+  ussd-pg:
+    driver: local

dev/init_db.sql

@@ -0,0 +1 @@
+CREATE DATABASE urdt_ussd;