use sh syntax

Author: aptalca

Dockerfile

@@ -7,7 +7,7 @@ ARG MOD_VERSION
 
 RUN \
   echo "**** retrieve latest version ****" && \
-  if [[ -z "${MOD_VERSION+x}" ]]; then \
+  if [ -z "${MOD_VERSION+x}" ]; then \
     DOCKER_RELEASE=$(curl -sX GET "https://api.github.com/repos/moby/moby/releases/latest" \
       | awk '/tag_name/{print $4;exit}' FS='[""]' \
       | sed 's|^v||'); \
@@ -26,7 +26,7 @@ RUN \
   BUILDX_RELEASE=$(curl -sX GET "https://api.github.com/repos/docker/buildx/releases/latest" \
     | awk '/tag_name/{print $4;exit}' FS='[""]') && \
   mkdir -p /root-layer/docker-tgz && \
-  if [[ $(uname -m) == "x86_64" ]]; then \
+  if [ $(uname -m) = "x86_64" ]; then \
     echo "**** grab x86_64 tarballs and binaries ****" && \
     curl -fo \
       /root-layer/docker-tgz/docker.tgz -L \
@@ -40,7 +40,7 @@ RUN \
     curl -fo \
       /root-layer/docker-tgz/docker-buildx -L \
       "https://github.com/docker/buildx/releases/download/${BUILDX_RELEASE}/buildx-${BUILDX_RELEASE}.linux-amd64"; \
-  elif [[ $(uname -m) == "aarch64" ]]; then \
+  elif [ $(uname -m) = "aarch64" ]; then \
     echo "**** grab aarch64 tarballs and binaries ****" && \
     curl -fo \
       /root-layer/docker-tgz/docker.tgz -L \