buid/test/docs: Docker image, Darwin fix, logo

.env.example

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+PARRHESIA_IMAGE=parrhesia:latest
+PARRHESIA_HOST_PORT=4000
+
+POSTGRES_DB=parrhesia
+POSTGRES_USER=parrhesia
+POSTGRES_PASSWORD=parrhesia
+
+DATABASE_URL=ecto://parrhesia:parrhesia@db:5432/parrhesia
+POOL_SIZE=20
+
+# Optional runtime overrides:
+# PARRHESIA_RELAY_URL=ws://localhost:4000/relay
+# PARRHESIA_POLICIES_AUTH_REQUIRED_FOR_WRITES=false
+# PARRHESIA_POLICIES_AUTH_REQUIRED_FOR_READS=false
+# PARRHESIA_POLICIES_MIN_POW_DIFFICULTY=0
+# PARRHESIA_FEATURES_VERIFY_EVENT_SIGNATURES=true
+# PARRHESIA_METRICS_ENABLED_ON_MAIN_ENDPOINT=true
+# PARRHESIA_METRICS_PRIVATE_NETWORKS_ONLY=true
+# PARRHESIA_METRICS_AUTH_TOKEN=
+# PARRHESIA_EXTRA_CONFIG=/config/parrhesia.runtime.exs