make build concurrency build-time customizable

scripts/Dockerfile

@@ -12,11 +12,27 @@ RUN ./scripts/install_deps.sh
 #Copy working directory on travis to the image
 COPY / $WORKDIR
 
+# Number of parallel jobs during build
+# or 0 for auto-computing (max(1, CPU_core_count * 2/3), a greedy value)
+ARG BUILD_CONCURRENCY="0"
+
 #Install dependencies, eliminate annoying warnings
 RUN sed -i -E -e 's/include <sys\/poll.h>/include <poll.h>/' /usr/include/boost/asio/detail/socket_types.hpp
 RUN cmake -DCMAKE_BUILD_TYPE=Release -DTESTS=0 -DSOLC_LINK_STATIC=1
-RUN make solc -j$(grep -c ^processor /proc/cpuinfo) && install -s  solc/solc /usr/bin
-#RUN strip solc/solc
+RUN make solc \
+    -j$(awk "BEGIN {                                       \
+        if (${BUILD_CONCURRENCY} != 0) {                   \
+            print(${BUILD_CONCURRENCY});                   \
+        } else {                                           \
+            x=($(grep -c ^processor /proc/cpuinfo) * 2/3); \
+            if (x > 1) {                                   \
+                printf(\"%d\n\", x);                       \
+            } else {                                       \
+                print(1);                                  \
+            }                                              \
+        }                                                  \
+    }")
+RUN strip solc/solc
 
 FROM scratch
 COPY --from=build /solidity/solc/solc /usr/bin/solc