Add entrypoint file

entrypoint.sh.tmpl

@@ -0,0 +1,29 @@
+#! /bin/bash
+
+set -eu
+
+file_env() {
+  local var="$1"
+  local fileVar="${var}_FILE"
+  local def="${2:-}"
+
+  if [ "${!var:-}" ] && [ "${!fileVar:-}" ]; then
+    echo >&2 "error: both $var and $fileVar are set (but are exclusive)"
+    exit 1
+  fi
+
+  local val="$def"
+  if [ "${!var:-}" ]; then
+    val="${!var}"
+  elif [ "${!fileVar:-}" ]; then
+    val="$(< "${!fileVar}")"
+  fi
+
+  export "$var"="$val"
+  unset "$fileVar"
+}
+
+file_env "MASTODON_ACCESS_TOKEN"
+
+echo "Passing it back to the upstream ENTRYPOINT/CMD..."
+exec "$@"