[FAB-8489] Make IMAGE_TAG optional

Existing sample documentation assumes that 'docker-compose' may be run
directly in the first-network directory.  Now that IMAGE_TAG is
parameterized, this creates failures if the var is unset.

This CR adds a default (which may be overridden via the environment) so
that this breakage does not occur.

Change-Id: Ibea4a3b3f823dfd3fc0e1eca3ab824b8230a8a41
Signed-off-by: Jason Yellick <jyellick@us.ibm.com>

first-network/.env

@@ -1 +1,2 @@
 COMPOSE_PROJECT_NAME=net
+IMAGE_TAG=latest