Temporarily use a non-default branch of cisagov/domain-scan

This is being done to test the changes in cisagov/domain-scan#6.  This
change can be reverted once cisagov/domain-scan#6 is approved and
merged.

Dockerfile

@@ -70,7 +70,7 @@ RUN pip3 install --no-cache-dir --upgrade \
 ###
 SHELL ["/bin/bash", "-o", "pipefail", "-c"]
 RUN mkdir ${CISA_HOME}/domain-scan \
-    && curl --location https://github.com/cisagov/domain-scan/tarball/master \
+    && curl --location https://github.com/cisagov/domain-scan/tarball/improvement/get-rid-of-regex-warnings \
     | tar --extract --gzip --strip-components 1 --directory ${CISA_HOME}/domain-scan/
 RUN pip3 install --no-cache-dir --upgrade \
     --requirement ${CISA_HOME}/domain-scan/requirements.txt