fix: provide default values for variables

entrypoint.sh.tmpl

@@ -22,7 +22,7 @@ file_env() {
 }
 
 load_vars() {
-  if [ -n "${CMD_DB_PASSWORD}" ] ; then
+  if [ -n "${CMD_DB_PASSWORD:-""}" ] ; then
     file_env "CMD_DB_PASSWORD"
   fi
   file_env "CMD_OAUTH2_CLIENT_SECRET"
@@ -30,7 +30,7 @@ load_vars() {
 }
 
 main() {
-  set -e
+  set -eu
 
   load_vars
   mkdir -p "/hedgedoc/.npm" && \
@@ -42,7 +42,7 @@ main() {
 
 main
 
-if [ -z "${CMD_DB_URL}" ] ; then
+if [ -z "${CMD_DB_URL:-""}" ] ; then
   # TODO: support databases other than postgres and sqlite
   export CMD_DB_URL="${postgres://$CMD_DB_USER:$CMD_DB_PASSWORD@$CMD_DB_HOST:5432/$CMD_DB_NAME}"
 fi
@@ -55,4 +55,4 @@ if [ ! "${1-}" == "-e" ]; then
   /usr/local/bin/docker-entrypoint.sh npm start
 fi
 
-set +e
+set +eu