docker: include git hash in Docker image.

Put the Git hash of the current commit inside the Docker image built by
Docker Hub. The hash is stored in the REVISION file in the root directory.

config/docker/Dockerfile.danbooru

@@ -47,6 +47,9 @@ RUN yarn install
 
 COPY . .
 
+ARG SOURCE_COMMIT
+RUN echo "$SOURCE_COMMIT" > REVISION
+
 
 
 FROM development AS assets