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>

.dockerignore

@@ -0,0 +1,13 @@
+/**
+!/cmd/africastalking
+!/common
+!/config
+!/initializers
+!/internal
+!/models
+!/remote
+!/services
+!/LICENSE
+!/README.md
+!/go.*
+!/.env