[FIX] Dockerfile: set m2 config directory

The upstream maven Docker image copies some files into the Maven config
directory during container startup, to fix some issues for images using
Maven at container runtime (see carlossg/docker-maven#11).

This broke startup for child images that do not run as root user (we're
adding a basex user). The same issue was described in
carlossg/docker-maven#14.

Resolves #1345, replaces #1346.

Dockerfile

@@ -9,16 +9,18 @@ RUN cd /usr/src/basex && \
     cp -r /usr/src/basex/basex-api/src/main/webapp/WEB-INF /srv
 
 # Run as non-privileged user with fixed UID
-RUN adduser --system --home /srv --disabled-password --disabled-login --uid 1984 basex && \
-    mkdir /srv/BaseXData /srv/BaseXRepo /srv/BaseXWeb && \
+# $MAVEN_CONFIG is expected to point to a volume by the parent maven image
+RUN adduser --home /srv --disabled-password --disabled-login --uid 1984 basex && \
+    mkdir /srv/.m2 /srv/BaseXData /srv/BaseXRepo /srv/BaseXWeb && \
     chown -R basex /srv
 USER basex
+ENV MAVEN_CONFIG=/srv/.m2
 
 # 1984/tcp: API
 # 8984/tcp: HTTP
 # 8985/tcp: HTTP stop
 EXPOSE 1984 8984 8985
-VOLUME ["/srv/BaseXData"]
+VOLUME ["/srv/.m2", "/srv/BaseXData"]
 WORKDIR /srv
 
 # Run BaseX HTTP server by default, logging to STDOUT