docker_run respect PX4_DOCKER_REPO env variable

Tools/docker_run.sh

 #! /bin/bash
 
-if [[ $@ =~ .*px4fmu.* ]]; then
-	# nuttx-px4fmu-v{1,2,3,4,5}
-	PX4_DOCKER_REPO="px4io/px4-dev-nuttx:2017-07-27"
-elif [[ $@ =~ .*rpi.* ]] || [[ $@ =~ .*bebop.* ]]; then
-	# posix_rpi_cross, posix_bebop_default
-	PX4_DOCKER_REPO="px4io/px4-dev-raspi:2017-07-27"
-elif [[ $@ =~ .*eagle.* ]] || [[ $@ =~ .*excelsior.* ]]; then
-	# eagle, excelsior
-	PX4_DOCKER_REPO="lorenzmeier/px4-dev-snapdragon:2017-07-28"
-elif [[ $@ =~ .*clang.* ]] || [[ $@ =~ .*scan-build.* ]]; then
-	# clang tools
-	PX4_DOCKER_REPO="px4io/px4-dev-clang:2017-07-27"
-elif [[ $@ =~ .*cppcheck.* ]]; then
-	# need Ubuntu 17.04 for cppcheck cmake support
-	# TODO: remove this once px4io/px4-dev-base updates
-	PX4_DOCKER_REPO=px4io/px4-dev-base:ubuntu17.04
-elif [[ $@ =~ .*tests* ]]; then
-	# run all tests with simulation
-	PX4_DOCKER_REPO="px4io/px4-dev-simulation:2017-07-27"
+if [ -z ${PX4_DOCKER_REPO+x} ]; then
+	echo "guessing PX4_DOCKER_REPO based on input";
+	if [[ $@ =~ .*px4fmu.* ]]; then
+		# nuttx-px4fmu-v{1,2,3,4,5}
+		PX4_DOCKER_REPO="px4io/px4-dev-nuttx:2017-07-27"
+	elif [[ $@ =~ .*rpi.* ]] || [[ $@ =~ .*bebop.* ]]; then
+		# posix_rpi_cross, posix_bebop_default
+		PX4_DOCKER_REPO="px4io/px4-dev-raspi:2017-07-27"
+	elif [[ $@ =~ .*eagle.* ]] || [[ $@ =~ .*excelsior.* ]]; then
+		# eagle, excelsior
+		PX4_DOCKER_REPO="lorenzmeier/px4-dev-snapdragon:2017-07-28"
+	elif [[ $@ =~ .*clang.* ]] || [[ $@ =~ .*scan-build.* ]]; then
+		# clang tools
+		PX4_DOCKER_REPO="px4io/px4-dev-clang:2017-07-27"
+	elif [[ $@ =~ .*cppcheck.* ]]; then
+		# need Ubuntu 17.04 for cppcheck cmake support
+		# TODO: remove this once px4io/px4-dev-base updates
+		PX4_DOCKER_REPO=px4io/px4-dev-base:ubuntu17.04
+	elif [[ $@ =~ .*tests* ]]; then
+		# run all tests with simulation
+		PX4_DOCKER_REPO="px4io/px4-dev-simulation:2017-07-27"
+	fi
+else
+	echo "PX4_DOCKER_REPO is set to '$PX4_DOCKER_REPO'";
 fi
 
 # otherwise default to nuttx
-if [ -z "$PX4_DOCKER_REPO" ]; then
+if [ -z ${PX4_DOCKER_REPO+x} ]; then
 	PX4_DOCKER_REPO="px4io/px4-dev-nuttx:2017-07-27"
 fi