Update create_image script for formal verification to fetch the corre…

…ct Dockerfile (#1281)

A recent AWS-LC-verification commit, awslabs/aws-lc-verification@7d4ee82, divides a single Dockerfile into two (Dockerfile.saw for the SAW proofs and Dockerfile.coq for the Coq proofs). Dockerfile file is no longer used. 
This commit updates the create_image.sh script for formal verification to use the updated file Dockerfile.saw.

tests/ci/docker_images/linux-x86/ubuntu-20.04_clang-10x_formal-verification/create_image.sh

@@ -10,6 +10,6 @@ fi
 rm -rf aws-lc-verification
 git clone https://github.com/awslabs/aws-lc-verification.git
 cd aws-lc-verification
-docker build --pull --no-cache -t ${docker_tag} .
+docker build --pull --no-cache -f Dockerfile.saw -t ${docker_tag} .
 cd ..
 rm -rf aws-lc-verification