Fix variable reference syntax

Author: Matthew-Whitlock

.github/docker-compose.yml

@@ -8,7 +8,7 @@ services:
         ARG OMPI_VERSION
         ENV OMPI_VERSION=$${OMPI_VERSION}
         CMD cp /configs/spack.yaml . && \
-            export OMPI_VERSION=$( [[ "$OMPI_VERSION" == git.* ]] && echo "${OMPI_VERSION}=main" || echo "${OMPI_VERSION}" ) \
+            export OMPI_VERSION=$( [[ "$${OMPI_VERSION}" == git.* ]] && echo "$${OMPI_VERSION}=main" || echo "$${OMPI_VERSION}" ) \
             spack -e . add openmpi@$${OMPI_VERSION} && \
             spack -e . containerize >/configs/spack.Dockerfile
       args: