  cp version.template build/version.txt

  sed -i "s/@VERSION@/${COMMIT_TAG:-dev}/" build/version.txt
  sed -i "s/@BUILD_DATE@/${BUILD_DATE}/" build/version.txt

Eww! Mutating a file in place, and needing a GNU extension for it.

   sed \
     -e "s/@VERSION@/${COMMIT_TAG:-dev}/" \
     -e "s/@BUILD_DATE@/${BUILD_DATE}/" \
     < build/version.template > build/version.txt