Add dockerignore to clean up final image (#384)

## Points for consideration:

Currently, the final base image has a bunch of things it doesn't need,
including the entire .git folder.

This commit adds a .dockerignore to keep the things we don't need out of
the final image.

## What's changed?

- fewer unneeded files in the final built image

---------

Co-authored-by: Adrian Lansdown <adrian.lansdown@raspberrypi.org>

Author: lpmi-13

.dockerignore

@@ -0,0 +1,45 @@
+# Version control
+.git
+.gitattributes
+.github
+.gitignore
+
+# CI/CD
+.circleci
+.clabot
+
+# Docker (no need to include itself)
+Dockerfile
+.dockerignore
+docker-compose.yml
+docker-compose.override.yml.example
+
+# Editor / IDE
+.vscode
+.devcontainer
+
+# Dev tooling
+.rspec
+.pryrc
+.ruby-lsp
+.DS_Store
+.rubocop.yml
+.rubocop_todo.yml
+.rubocop-https--*
+
+# Documentation
+*.md
+LICENSE
+COPYRIGHT
+docs
+
+# Environment
+.env.example
+
+# Tests
+spec
+
+# Runtime dirs (recreated at runtime)
+log
+tmp
+storage