Fix Martin config to inject DATABASE_URL at startup via envsubst

Martin ignores DATABASE_URL env var when a config file is loaded,
so we use envsubst to template the connection string into the config.

Author: mberg

truecover-backend/Dockerfile.martin

@@ -1,10 +1,13 @@
 # ABOUTME: Docker build for Martin tile server on Railway.
-# ABOUTME: Copies config and serves tiles from PostGIS and /pmtiles volume.
+# ABOUTME: Copies config template and substitutes DATABASE_URL at startup.
 
 FROM ghcr.io/maplibre/martin:latest
 
-COPY martin-railway.yaml /config/config.yaml
+USER root
+RUN apk add --no-cache gettext
+
+COPY martin-railway.yaml /config/config.yaml.template
 
 EXPOSE 3000
 
-CMD ["--config", "/config/config.yaml"]
+ENTRYPOINT ["sh", "-c", "envsubst < /config/config.yaml.template > /config/config.yaml && martin --config /config/config.yaml"]

truecover-backend/martin-railway.yaml

@@ -5,6 +5,7 @@ cache_size_mb: 512
 preferred_encoding: gzip
 
 postgres:
+  connection_string: ${DATABASE_URL}
   auto_publish: true
 
   functions: