Update deps and Alpine image

- Updated deps
- Updated Alpine images to 3.16
- Removed dumb-init, not needed anymore
- Some small shellcheck tweaks on the start/healthcheck scripts

docker/healthcheck.sh

@@ -2,8 +2,8 @@
 
 # Use the value of the corresponding env var (if present),
 # or a default value otherwise.
-: ${DATA_FOLDER:="data"}
-: ${ROCKET_PORT:="80"}
+: "${DATA_FOLDER:="data"}"
+: "${ROCKET_PORT:="80"}"
 
 CONFIG_FILE="${DATA_FOLDER}"/config.json