Fix SQLite support (#210)

SQLite DB was not writable because it was always located in
/usr/src/shynet/, which is owned by root and not writable by
appuser. SQLite needs the parent directory containing the DB file to be
writable by the running user.

The applied fix is to place the DB file in /var/local/shynet/db and to
create that directory in the Docker image with the right permissions.

SQLite setup is now documented in README as an alternative to Postgres.

Dockerfile

@@ -37,7 +37,9 @@ RUN apk --purge del .build-deps && \
 	rm -rf /var/lib/apt/lists/* && \
 	rm /var/cache/apk/* && \
 	addgroup --system -g $GF_GID appgroup && \
-	adduser appuser --system --uid $GF_UID -G appgroup
+	adduser appuser --system --uid $GF_UID -G appgroup && \
+	mkdir -p /var/local/shynet/db/ && \
+	chown -R appuser:appgroup /var/local/shynet
 
 # Install Shynet
 COPY shynet .