docker: update deprecated syntax

`docker build` warns of deprecated syntax:

    1 warning found (use docker --debug to expand):
    - LegacyKeyValueFormat: "ENV key=value" should be used instead of legacy "ENV key value" format (line 71)

Update Dockerfile, removing the deprecated syntax usage.

Author: WhyNotHugo

docker/Dockerfile

@@ -68,5 +68,5 @@ RUN adduser infinitime
 # Configure Git to accept the /sources directory as safe
 RUN git config --global --add safe.directory /sources
 
-ENV SOURCES_DIR /sources
+ENV SOURCES_DIR=/sources
 CMD ["/opt/build.sh"]