Redo docker build system with buildkit+multi-stage and cache pio packages (#2338)

docker/docker_entrypoint.sh

@@ -0,0 +1,18 @@
+#!/bin/bash
+
+# If /cache is mounted, use that as PIO's coredir
+# otherwise use path in /config (so that PIO packages aren't downloaded on each compile)
+
+if [[ -d /cache ]]; then
+    export PLATFORMIO_CORE_DIR=/cache/platformio
+else
+    export PLATFORMIO_CORE_DIR=/config/.esphome/platformio
+fi
+
+if [[ ! -d "${PLATFORMIO_CORE_DIR}" ]]; then
+    echo "Creating cache directory ${PLATFORMIO_CORE_DIR}"
+    echo "You can change this behavior by mounting a directory to the container's /cache directory."
+    mkdir -p "${PLATFORMIO_CORE_DIR}"
+fi
+
+exec esphome "$@"