diff --git a/.tool-versions b/.tool-versions
index 4933a41..83d3f70 100644
--- a/.tool-versions
+++ b/.tool-versions
@@ -1 +1 @@
-golang 1.18
+golang 1.21.4
diff --git a/Dockerfile b/Dockerfile
index 791a922..a444666 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@sha256:96a8a701943e7eabd81ebd0963540ad660e29c3b2dc7fb9d7e06af34409e9ba6 as builder
 
 RUN apk add --no-cache git build-base
 COPY . /doctree