Updated README.

README.md

@@ -121,7 +121,8 @@ of this parameter has the format `<VARIABLE_NAME>=<VALUE>`.
 |`TZ`| [TimeZone](http://en.wikipedia.org/wiki/List_of_tz_database_time_zones) used by the container.  Timezone can also be set by mapping `/etc/localtime` between the host and the container. | `Etc/UTC` |
 |`KEEP_APP_RUNNING`| When set to `1`, the application will be automatically restarted when it crashes or terminates. | `0` |
 |`APP_NICENESS`| Priority at which the application should run.  A niceness value of -20 is the highest priority and 19 is the lowest priority.  The default niceness value is 0.  **NOTE**: A negative niceness (priority increase) requires additional permissions.  In this case, the container should be run with the docker option `--cap-add=SYS_NICE`. | `0` |
-|`INSTALL_PACKAGES`| Space-separated list of packages to install during the startup of the container.  Packages are installed from the repository of the Linux distribution this container is based on.  **ATTENTION**: Container functionality can be affected when installing a package that overrides existing container files (e.g. binaries). | (no value) |
+|`INSTALL_PACKAGES`| Space-separated list of packages to install during the startup of the container.  List of available packages can be found at https://mirrors.alpinelinux.org.  **ATTENTION**: Container functionality can be affected when installing a package that overrides existing container files (e.g. binaries). | (no value) |
+|`PACKAGES_MIRROR`| Mirror of the repository to use when installing packages. List of mirrors is available at https://mirrors.alpinelinux.org. | (no value) |
 |`CONTAINER_DEBUG`| Set to `1` to enable debug logging. | `0` |
 |`DISPLAY_WIDTH`| Width (in pixels) of the application's window. | `1920` |
 |`DISPLAY_HEIGHT`| Height (in pixels) of the application's window. | `1080` |