Build image only during build

Signed-off-by: Pol Henarejos <pol.henarejos@cttc.es>

tests/build-in-docker.sh

@@ -1,6 +1,7 @@
 #!/bin/bash -eu
 
 source tests/docker_env.sh
+build_image
 #run_in_docker rm -rf CMakeFiles
 run_in_docker mkdir -p build_in_docker
 run_in_docker -w "$PWD/build_in_docker" cmake -DENABLE_EMULATION=1 ..

tests/docker_env.sh

@@ -72,14 +72,16 @@ else
     NUM_PROC="$(nproc)"
 fi
 
-# Build the Docker image
-echo "Getting docker image up to date (this may take a few minutes)..."
-${DOCKER} image build \
+build_image() {
+    # Build the Docker image
+    echo "Getting docker image up to date (this may take a few minutes)..."
+    ${DOCKER} image build \
         -t ${DOCKER_IMAGE_TAG} \
         --cache-from=${DOCKER_IMAGE_TAG} \
         --network host \
         --build-arg MAKEFLAGS_PARALLEL="-j ${NUM_PROC}" \
         tests/docker/${MBEDTLS_DOCKER_GUEST}
+}
 
 run_in_docker()
 {