fix: use herokuish image digest when computing the sha of the apt commands

Previously, we would take the herokuish image as is, meaning we would sometimes have stale base images when herokuish was upgraded. This new path avoids this by taking a sha of the base image, and then re-applying the apt commands as expected.

Closes #48

Author: josegonzalez

internal-functions

@@ -40,7 +40,10 @@ fn-apt-fetch-sha() {
     return
   fi
 
-  echo -n "$(<$PLUGIN_AVAILABLE_PATH/apt/plugin.toml)${DOKKU_IMAGE}${CONTENT}" | sha256sum | cut -d " " -f 1
+  local image_digest
+  image_digest="$("$DOCKER_BIN" image inspect --format='{{index .RepoDigests 0}}' "$DOKKU_IMAGE" 2>/dev/null || echo "$DOKKU_IMAGE")"
+
+  echo -n "$(<$PLUGIN_AVAILABLE_PATH/apt/plugin.toml)${image_digest}${CONTENT}" | sha256sum | cut -d " " -f 1
 }
 
 fn-apt-populate-work-dir() {