Fix entrypoint script

A missing space in line 4 broke some important functionality for the
entrypoint script. Additionally in order to find further problems,
running shellcheck was used to find some other minor issues which are
fixed in this patch as well.

resources/docker-entrypoint.sh

 #!/bin/sh
 
 # Use gosu if the container started with root privileges
-UID=$(id -u)
-[ $UID -eq 0] && GOSU="gosu codimd" || GOSU=""
+UID="$(id -u)"
+[ "$UID" -eq 0 ] && GOSU="gosu codimd" || GOSU=""
 
 if [ "$HMD_DB_URL" != "" ] && [ "$CMD_DB_URL" = "" ]; then
     CMD_DB_URL="$HMD_DB_URL"
@@ -21,7 +21,7 @@ export CMD_DB_URL
 DB_SOCKET=$(echo ${CMD_DB_URL} | sed -e 's/.*:\/\//\/\//' -e 's/.*\/\/[^@]*@//' -e 's/\/.*$//')
 
 if [ "$DB_SOCKET" != "" ]; then
-    dockerize -wait tcp://${DB_SOCKET} -timeout 30s
+    dockerize -wait "tcp://${DB_SOCKET}" -timeout 30s
 fi
 
 $GOSU ./node_modules/.bin/sequelize db:migrate
@@ -45,8 +45,8 @@ $GOSU ./node_modules/.bin/sequelize db:migrate
 } ; }
 
 # Change owner and permission if filesystem backend is used and user has root permissions
-if [ $UID -eq 0 && "$CMD_IMAGE_UPLOAD_TYPE" = "filesystem" ]; then
-    if [ $UID -eq 0 ]; then
+if [ "$UID" -eq 0 ] && [ "$CMD_IMAGE_UPLOAD_TYPE" = "filesystem" ]; then
+    if [ "$UID" -eq 0 ]; then
         chown -R codimd ./public/uploads
         chmod 700 ./public/uploads
     else