Support loading env vars from .env files in container

README.md

@@ -39,6 +39,10 @@ services:
 
 Certain values can be set via environment variables, using the `-e` parameter on the docker command line, or the `environment:` section in docker-compose.
 
+You can also load variables from files:
+* Docker CLI supports `--env-file /path/to/.env`.
+* The container entrypoint auto-loads `/app/.env` if present, and also loads a custom file when `ENV_FILE` points to it.
+
 ### ⬇️ Download Behavior
 
 * __MAX_CONCURRENT_DOWNLOADS__: Maximum number of simultaneous downloads allowed. For example, if set to `5`, then at most five downloads will run concurrently, and any additional downloads will wait until one of the active downloads completes. Defaults to `3`. 

docker-entrypoint.sh

@@ -1,5 +1,21 @@
 #!/bin/sh
 
+load_env_file() {
+    env_file="$1"
+    if [ -f "$env_file" ]; then
+        echo "Loading environment from ${env_file}"
+        # shellcheck disable=SC1090
+        set -a
+        . "$env_file"
+        set +a
+    fi
+}
+
+load_env_file "/app/.env"
+if [ -n "${ENV_FILE}" ]; then
+    load_env_file "${ENV_FILE}"
+fi
+
 PUID="${UID:-$PUID}"
 PGID="${GID:-$PGID}"