don't skip install phase inside docker

ci/install.sh

@@ -62,7 +62,7 @@ EOF
 }
 
 main() {
-    if [[ -z $DOCKER ]]; then
+    if [[ ${DOCKER:-n} != "y" ]]; then
         install_qemu
         install_binutils
         install_c_toolchain

ci/script.sh

@@ -39,7 +39,8 @@ main() {
                -e TRAVIS_OS_NAME=$TRAVIS_OS_NAME \
                -v $(pwd):/mnt \
                ubuntu:16.04 \
-               sh -c 'cd /mnt;
+               sh -c 'set -ex;
+                      cd /mnt;
                       export PATH="$PATH:$HOME/.cargo/bin";
                       bash ci/install.sh;
                       bash ci/script.sh'