docker: widen build context to repo root for build-time install stage

The Dockerfile will need access to chatmaild/ and cmdeploy/ source
trees to run CMDEPLOY_STAGES=install via pyinfra during image build,
moving install-time work out of container startup. The previous context
(./docker) only included helper scripts.

Also adds .dockerignore to exclude .git, data/, venv/ etc. from the
build context, and updates COPY paths accordingly.

docker-compose.yaml

@@ -1,8 +1,8 @@
 services:
   chatmail:
     build:
-      context: ./docker
-      dockerfile: chatmail_relay.dockerfile
+      context: ./
+      dockerfile: docker/chatmail_relay.dockerfile
       tags:
         - chatmail-relay:latest
     image: chatmail-relay:latest