Keep the working tree out of the image build context

Without a .dockerignore a build from the developer machine sends the 19 GB
corpus, the virtualenv and node_modules to the daemon; the Dockerfile reads
none of them.

.dockerignore

@@ -0,0 +1,18 @@
+.git
+.venv
+**/node_modules
+**/__pycache__
+*.pyc
+.mypy_cache
+.ruff_cache
+.pytest_cache
+.pi
+.ely_eye
+artifacts
+bench/corpus
+bench/runs
+bench/sets/corpus
+frontend/dist
+.github
+Gaps.md
+CHECKLIST.md