Docker: Add .env

This file was copied from
https://github.com/OpenSlides/openslides-docker-compose/
and still needs to be adjusted for the present setup.

docker/.env

@@ -0,0 +1,47 @@
+# OpenSlides instance configuration
+#
+# As well as environment variables for various services, this file contains
+# variables used to persist custom settings for docker-compose.yml or
+# docker-stack.yml.  See the preamble of a docker-compose.yml.m4 or
+# docker-stack.yml.m4 template for more information.
+#
+# Most variables are listed here only to facilitate discovery of the available
+# options.  Empty values cause the template's defaults to be inserted.
+
+# General
+# -------
+INSTANCE_DOMAIN=
+PROJECT_STACK_NAME=
+EXTERNAL_HTTP_PORT=
+DEFAULT_DOCKER_REGISTRY=
+
+# Docker Images
+# -------------
+DOCKER_OPENSLIDES_BACKEND_NAME=
+DOCKER_OPENSLIDES_BACKEND_TAG=
+DOCKER_OPENSLIDES_FRONTEND_NAME=
+DOCKER_OPENSLIDES_FRONTEND_TAG=
+
+# Database
+# --------
+PGNODE_2_ENABLED=
+PGNODE_3_ENABLED=
+PGNODE_REPMGR_PRIMARY=
+PGNODE_WAL_ARCHIVING=
+PGNODE_1_PLACEMENT_CONSTR=
+PGNODE_2_PLACEMENT_CONSTR=
+PGNODE_3_PLACEMENT_CONSTR=
+PGBOUNCER_PLACEMENT_CONSTR=
+
+# Service Replication
+# -------------------
+OPENSLIDES_BACKEND_SERVICE_REPLICAS=
+OPENSLIDES_FRONTEND_SERVICE_REPLICAS=
+REDIS_RO_SERVICE_REPLICAS=
+MEDIA_SERVICE_REPLICAS=
+
+# E-Mail
+# ------
+DEFAULT_FROM_EMAIL=
+POSTFIX_MYHOSTNAME=
+POSTFIX_RELAYHOST=