Fix all hadolint warnings in Dockerfile (#580)

* Fix all hadolint warnings in Dockerfile

- Double-quote variable reference in RUN sed command (SC2086)
- Use absolute paths for COPY destinations (DL3045)

* Revert quoting on line 18 (will be removed in composer.local.json PR)

Author: cicalese

Dockerfile

@@ -14,6 +14,6 @@ ENV MW_MAINTENANCE_CIRRUSSEARCH_UPDATECONFIG=2 \
 	MW_MAINTENANCE_CIRRUSSEARCH_FORCEINDEX=2
 
 # Maintenance scripts for specific extensions
-COPY cirrus-search-maintenance.sh _sources/scripts/maintenance-scripts/
-COPY getSMWSettings.php _sources/canasta/
-COPY smw-maintenance.sh _sources/scripts/maintenance-scripts/
+COPY cirrus-search-maintenance.sh /_sources/scripts/maintenance-scripts/
+COPY getSMWSettings.php /_sources/canasta/
+COPY smw-maintenance.sh /_sources/scripts/maintenance-scripts/