docker fix typo in env variable

fixes #98

Author: lovasoa

Dockerfile

@@ -32,7 +32,7 @@ RUN touch src/main.rs && \
 FROM busybox:glibc
 RUN addgroup --system sqlpage && \
     adduser --system --no-create-home --ingroup sqlpage sqlpage
-ENV SQLPAGE_ROOT=/var/www
+ENV SQLPAGE_WEB_ROOT=/var/www
 WORKDIR /etc
 COPY --from=builder /usr/src/sqlpage/sqlpage.bin /usr/local/bin/sqlpage
 COPY --from=builder /usr/src/sqlpage/libgcc_s.so.1 /lib/libgcc_s.so.1