Update dic location.

validation/Dockerfile

@@ -89,7 +89,9 @@ RUN set -e pipefail; \
     export _DICT_DIR="${SRC_DIR}/mmcif_dicts"; \
     export _DICT_URL="https://mmcif.wwpdb.org/dictionaries/ascii"; \
     export _GIT_URL="https://raw.github.com/ihmwg/ModelCIF/master"; \
-    export _MA_DICT_URL="${_GIT_URL}/archive/mmcif_ma-v${USE_DICT_VERSION}.dic"; \
+    # Use the path of an actual commit to keep the dict immutable (RCSB refuses
+    # to use Git tags for versions).
+    export _MA_DICT_URL="https://raw.githubusercontent.com/ihmwg/ModelCIF/ba728c4/archive/mmcif_ma-v${USE_DICT_VERSION}.dic"; \
     mkdir ${_DICT_DIR}; \
     mkdir ${MMCIF_DICTS_DIR}; \
     cd ${_DICT_DIR}; \