Install linux-headers in dockerfile

This is required to build psutil. Without this patch, I get:

psutil/_psutil_posix.c:29:29: fatal error: linux/types.h: No such file or directory

After this patch, 'docker build' completes successfully.

See also: giampaolo/psutil#872

Dockerfile

@@ -8,7 +8,7 @@ ENV SQLITE_DB=/data/vmprof.db
 
 RUN apk add --no-cache python3 \
         py3-yaml py3-cryptography py3-six py3-requests sqlite py-pysqlite libunwind-dev uwsgi-python3 \
-        gcc g++ musl-dev postgresql-dev python3-dev git
+        gcc g++ musl-dev linux-headers postgresql-dev python3-dev git
 
 COPY requirements /usr/src/requirements