No, cubemaps are not in the file already, you have to build them.
You need to do the following in your console:
If you have HDR lighting:
-load the map
-mat_specular 0
-buildcubemaps
-disconnect
-mat_hdr_level 0
-map <mapname>
-buildcubemaps
-disconnect
-mat_specular 1
-mat_hdr_level 2
-reload map and confirm reflections are working
If you don't have HDR lighting:
-load the map
-mat_specular 0
-buildcubemaps
-disconnect
-mat_specular 1
-reload map and confirm reflections are working
This will cycle through each of your cubemap entities in the map and take a quick picture from every direction. You do mat_specular 0 to turn off the current pink checkerboard reflections so they don't show up when they are in properly.
Afterwards you'll want to pack in your custom content, a common tool currently is
VIDE.
If you want to go the extra step, you can compress your map into a BZ2 file using 7zip and upload that as well. This is for server owners and saves them a step for putting your map on their server. A regular ZIP of the BSP file is for players who just want to load it up and run around, or predownload it.