Buidling `cybertec/pgwatch2` failed

[pashagolub]

$ docker build --no-cache --build-arg GIT_TIME=`git show -s --format=%cI HEAD` \
--build-arg GIT_HASH=`git show -s --format=%H HEAD` -t cybertec/pgwatch2:latest -f Dockerfile .
...
#18 14.33     building '_cffi_backend' extension
#18 14.33     creating build/temp.linux-x86_64-3.5
#18 14.33     creating build/temp.linux-x86_64-3.5/c
#18 14.33     x86_64-linux-gnu-gcc -pthread -DNDEBUG -g -fwrapv -O2 -Wall -Wstrict-prototypes -g -fstack-protector-strong -Wformat -Werror=format-security -Wdate-time -D_FORTIFY_SOURCE=2 -fPIC -DUSE__THREAD -DHAVE_SYNC_SYNCHRONIZE -I/usr/include/ffi -I/usr/include/libffi -I/usr/include/python3.5m -c c/_cffi_backend.c -o build/temp.linux-x86_64-3.5/c/_cffi_backend.o
#18 14.33     c/_cffi_backend.c:15:17: fatal error: ffi.h: No such file or directory
#18 14.33     compilation terminated.
#18 14.33     error: command 'x86_64-linux-gnu-gcc' failed with exit status 1
...