common/travis/docker.sh: add control variables

for more flexible local testing using existing tools

.travis.yml

@@ -7,6 +7,7 @@ git:
 env:
     global:
         - PATH=$PATH:/tmp/bin
+        - DOCKER_BASE=voidlinux/masterdir
         - TAG=2019042701
 
     matrix:

common/travis/docker.sh

@@ -4,13 +4,15 @@
 
 [ "$XLINT" ] && exit 0
 
-/bin/echo -e "\x1b[32mPulling docker image for $BOOTSTRAP from $TAG...\x1b[0m"
-docker pull voidlinux/masterdir-$BOOTSTRAP:$TAG
+DOCKER_NAME=${DOCKER_NAME:-void}
+
+/bin/echo -e "\x1b[32mPulling docker image $DOCKER_BASE-$BOOTSTRAP:$TAG...\x1b[0m"
+docker pull $DOCKER_BASE-$BOOTSTRAP:$TAG
 docker run -d \
-	   --name void \
+	   --name $DOCKER_NAME \
 	   -v "$(pwd)":/hostrepo \
 	   -v /tmp:/tmp \
 	   -e XLINT="$XLINT" \
 	   -e PATH="$PATH" \
-	   voidlinux/masterdir-$BOOTSTRAP:$TAG \
+	   $DOCKER_BASE-$BOOTSTRAP:$TAG \
 	   /bin/sh -c 'sleep inf'