chore(deps): pin dependencies

Change-Id: I8d76ec8d448956edd690aedf50acbbc598a1f16a
diff --git a/Dockerfile b/Dockerfile
index 9586a6a..8ef60dc 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -1,4 +1,4 @@
-FROM golang:1.18 as builder
+FROM golang:1.18@sha256:50c889275d26f816b5314fc99f55425fa76b18fcaf16af255f5d57f09e1f48da as builder
 
 ENV CGO_ENABLED=0
 
@@ -13,7 +13,7 @@
 
 RUN go install gomodules.avm99963.com/vulnzybot
 
-FROM alpine
+FROM alpine@sha256:51b67269f354137895d43f3b3d810bfacd3945438e94dc5ac55fdac340352f48
 LABEL maintainer "me@avm99963.com"
 
 COPY crontab /etc/crontabs/root