Improve builder script.

- Don't just pull all tags from the upstream registries. That's mental.
- Don't try and cross compile unless the arch is arm and the build platform is not arm.

build.sh

@@ -15,17 +15,20 @@ ERLANG_CONTAINER=$CI_REGISTRY/jimsy/balena-erlang/$ARCH-$VARIANT-build
 
 if (echo $ARCH | grep 'arm\|aarch64\|rpi')
 then
-  CROSS_BUILD_START='RUN [ "cross-build-start" ]'
+  if (uname -m | grep 'x86_64\|i386')
-  CROSS_BUILD_END='RUN [ "cross-build-start" ]'
+  then
+    CROSS_BUILD_START='RUN [ "cross-build-start" ]'
+    CROSS_BUILD_END='RUN [ "cross-build-end" ]'
+  else
+    CROSS_BUILD_START=''
+    CROSS_BUILD_END=''
+  fi
 else
   CROSS_BUILD_START=''
   CROSS_BUILD_END=''
 fi
 
 docker pull $BASE_CONTAINER || true
-docker pull -a $CONTAINER_NAME-build || true
-docker pull -a $CONTAINER_NAME || true
-docker pull -a $ERLANG_CONTAINER || true
 
 grep -v '^#' versions | sort -r | while IFS=' ' read ELIXIR_VERSION ERLANG_MAJOR_VERSION
 do
@@ -34,6 +37,13 @@ do
   ELIXIR_MAJOR_VERSION=$(echo $ELIXIR_VERSION | cut -d\. -f1-2)
   TEMP_CONTAINER=$ARCH-$VARIANT-build-$ELIXIR_VERSION-$TAG:$CI_COMMIT_SHA
 
+  docker pull $ERLANG_CONTAINER:$ERLANG_MAJOR_VERSION-$TAG || true
+  docker pull $CONTAINER_NAME-build-$ELIXIR_VERSION-$TAG || true
+  docker pull $CONTAINER_NAME-build-$ELIXIR_MAJOR_VERSION-$TAG || true
+  docker pull $CONTAINER_NAME-$ELIXIR_VERSION-$TAG || true
+  docker pull $CONTAINER_NAME-$ELIXIR_MAJOR_VERSION-$TAG || true
+
+
   sed \
     -e "s~#{CROSS_BUILD_START}~$CROSS_BUILD_START~g" \
     -e "s~#{CROSS_BUILD_END}~$CROSS_BUILD_END~g" \