add dockerignore (dagster-io#26075)

## Summary & Motivation

This lets us use the dagster repo as a docker context, without having to transfer a lot of cruft through docker that would then get ignored anyway

## How I Tested These Changes

Ran a docker build locally with /code/dagster as the context, and verified that before this change the context was unmanegeable (several GB) and after it was basically immediate

Author: neilfulwiler

.dockerignore

@@ -0,0 +1,9 @@
+.git/
+**/.tox/
+**/.yarn/
+**/__pycache__/
+**/*.egg-info/
+**/node_modules/
+**/*.pyc
+**/.next/
+**/venv/