build: Restore .dockerignore to improve image build and caching (#36877)

edx-platform's Dockerfile was removed in Jan 2025 because the
community was not maintaining or using it. The PR that removed it [1]
also removed the .dockerfile as an effort to clean up unnecessary
Docker-related files from the repo.

However, the .dockerignore file serves an important purpose for
*any* Docker image based on edx-platform: it identifies which files
and paths aren't relevant for the functioning of the image. For
example, Tutor greatly benefited from this file, because by ignoring
unnecessary files, the build context was shrunk and image cache
layers were less likely to be unnecesarily invalidated.

We restore the .dockerfile as it was previously in Sumac [2], with
the addition of /venv, /.venv, and __pycache__, and with the
removal of /Dockerfile (as it no longer exists).

[1] https://github.com/openedx/edx-platform/pull/35731
[2] https://github.com/openedx/edx-platform/blob/open-release/sumac.master/.dockerignore

.dockerignore

@@ -0,0 +1,153 @@
+# .dockerignore for edx-platform.
+# There's a lot here, please try to keep it organized.
+
+### Files that are not needed in the docker file
+
+/test_root/
+.git
+
+### Files private to developers
+
+# Files that should be git-ignored, but are hand-edited or otherwise valued,
+# and so should not be destroyed by "make clean".
+# start-noclean
+requirements/private.txt
+requirements/edx/private.in
+requirements/edx/private.txt
+lms/envs/private.py
+cms/envs/private.py
+# end-noclean
+
+### Python artifacts
+**/*.pyc
+**/__pycache__
+.venv
+venv
+
+### Editor and IDE artifacts
+**/*~
+**/*.swp
+**/*.orig
+**/nbproject
+**/.idea/
+**/.redcar/
+**/codekit-config.json
+**/.pycharm_helpers/
+**/_mac/*
+**/IntelliLang.xml
+**/conda_packages.xml
+**/databaseSettings.xml
+**/diff.xml
+**/debugger.xml
+**/editor.xml
+**/ide.general.xml
+**/inspection/Default.xml
+**/other.xml
+**/packages.xml
+**/web-browsers.xml
+
+### NFS artifacts
+**/.nfs*
+
+### OS X artifacts
+**/*.DS_Store
+**/.AppleDouble
+**/:2e_*
+**/:2e#
+
+### Internationalization artifacts
+**/*.mo
+**/*.po
+**/*.prob
+**/*.dup
+!**/django.po
+!**/django.mo
+!**/djangojs.po
+!**/djangojs.mo
+conf/locale/en/LC_MESSAGES/*.mo
+conf/locale/fake*/LC_MESSAGES/*.po
+conf/locale/fake*/LC_MESSAGES/*.mo
+
+### Testing artifacts
+**/.testids/
+**/.noseids
+**/nosetests.xml
+**/.cache/
+**/.coverage
+**/.coverage.*
+**/coverage.xml
+**/cover/
+**/cover_html/
+**/reports/
+**/jscover.log
+**/jscover.log.*
+**/.pytest_cache/
+**/pytest_task*.txt
+**/.tddium*
+common/test/data/test_unicode/static/
+test_root/courses/
+test_root/data/test_bare.git/
+test_root/export_course_repos/
+test_root/paver_logs/
+test_root/uploads/
+**/django-pyfs
+**/.tox/
+common/test/data/badges/*.png
+
+### Installation artifacts
+**/*.egg-info
+**/.pip_download_cache/
+**/.prereqs_cache
+**/.vagrant/
+**/node_modules
+**/bin/
+
+### Static assets pipeline artifacts
+**/*.scssc
+lms/static/css/
+lms/static/certificates/css/
+cms/static/css/
+common/static/common/js/vendor/
+common/static/common/css/vendor/
+common/static/bundles
+**/webpack-stats.json
+
+### Styling generated from templates
+lms/static/sass/*.css
+lms/static/sass/*.css.map
+lms/static/certificates/sass/*.css
+lms/static/themed_sass/
+cms/static/css/
+cms/static/sass/*.css
+cms/static/sass/*.css.map
+cms/static/themed_sass/
+themes/**/css
+
+### Logging artifacts
+**/log/
+**/logs
+**/chromedriver.log
+**/ghostdriver.log
+
+### Celery artifacts ###
+**/celerybeat-schedule
+
+### Unknown artifacts
+**/database.sqlite
+**/courseware/static/js/mathjax/*
+**/flushdb.sh
+**/build
+/src/
+\#*\#
+**/.env/
+openedx/core/djangoapps/django_comment_common/comment_client/python
+**/autodeploy.properties
+**/.ws_migrations_complete
+**/dist
+**/*.bak
+
+# Visual Studio Code
+**/.vscode
+
+# Locally generated PII reports
+**/pii_report