fix: docker image content

- [x] add .dockerignore to ensure non application files/test files don't get into the container image

Tested locally with the following commands:

```bash
➜ docker build . --tag test:1
➜ docker run -it --rm --name debug --entrypoint /bin/bash test:1

## inside container

root@3624b051f2c8:/action/workspace# ls -lah
total 44K
drwxr-xr-x 1 root root 4.0K May 27 13:11 .
drwxr-xr-x 1 root root 4.0K May 27 13:11 ..
-rw-r--r-- 1 root root 1.8K Jan  7 10:26 auth.py
-rw-r--r-- 1 root root 4.4K Mar 30 23:25 env.py
-rw-r--r-- 1 root root   68 Apr  4 04:04 requirements.txt
-rwxr-xr-x 1 root root  14K May 27 12:11 stale_repos.py
```

Signed-off-by: jmeridth <[email protected]>

Author: jmeridth

.dockerignore

@@ -0,0 +1,36 @@
+# Application specific filesAdd commentMore actions
+test_*.py
+
+# Python
+*.pyc
+__pycache__/
+*.pyo
+*.pyd
+
+# Common
+*.md
+docker-compose.yml
+Dockerfile*
+.env*
+Makefile
+
+# Logs
+logs
+*.log
+
+# IDE's
+.vscode/
+.idea/
+
+# Dependency directories
+node_modules/
+.venv/
+
+## Cache directories
+.parcel-cache
+
+# git
+.git
+.gitattributes
+.gitignore
+.github/