diff --git a/.tool-versions b/.tool-versions
index 4933a41..ff2d562 100644
--- a/.tool-versions
+++ b/.tool-versions
@@ -1 +1 @@
-golang 1.18
+golang 1.21.2
diff --git a/Dockerfile b/Dockerfile
index 791a922..773c4c5 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -2,7 +2,7 @@
 ###############################
 # Build the doctree Go binary #
 ###############################
-FROM golang:1.18-alpine as builder
+FROM golang:1.21-alpine@sha256:1c9cc949513477766da12bfa80541c4f24957323b0ee00630a6ff4ccf334b75b as builder
 
 RUN apk add --no-cache git build-base
 COPY . /doctree