docker/bootstrap: Replace ENV MAKEFLAGS with setting it per command.

The ENV version was not working anyway since Docker did single-quote: '-j$(nproc)'

Because of that make saw this string literal and converted it into an (unknown) absurd high number. On my 12 core machine it executed way more jobs in parallel than 12. This change will also fix this.

docker/bootstrap/Dockerfile.common

@@ -1,8 +1,5 @@
 FROM golang:1.5
 
-# Use all cores for parallel make (load bound to number of cores as well).
-ENV MAKEFLAGS -j$(nproc) -l$(nproc)
-
 # Install Vitess build dependencies
 RUN apt-get update && DEBIAN_FRONTEND=noninteractive apt-get install -y --no-install-recommends \
     automake \
@@ -36,7 +33,8 @@ RUN curl -sL --connect-timeout 10 --retry 3 \
     https://bootstrap.pypa.io/get-pip.py | python
 
 # Install PHP modules for running tests
-RUN mkdir -p /vt/bin && \
+RUN export MAKEFLAGS="-j$(nproc)" && \
+    mkdir -p /vt/bin && \
     curl -sL --connect-timeout 10 --retry 3 \
         https://phar.phpunit.de/phpunit-4.8.9.phar > /vt/bin/phpunit && \
     chmod +x /vt/bin/phpunit && \
@@ -87,7 +85,8 @@ COPY travis /vt/src/github.com/youtube/vitess/travis
 COPY composer.json composer.lock /vt/src/github.com/youtube/vitess/
 
 # Install gRPC (and protobuf) as root
-RUN cd /vt/dist && \
+RUN export MAKEFLAGS="-j$(nproc)" && \
+    cd /vt/dist && \
     INSTALL_GRPC_PHP=`php-config --extension-dir` $VTTOP/travis/install_grpc.sh && \
     echo 'extension=grpc.so' > /etc/php5/cli/conf.d/20-grpc.ini && \
     rm -rf /vt/dist/grpc