diff --git a/.github/workflows/adhoctest.yml b/.github/workflows/adhoctest.yml
index 4a355c8aa..065a9a5e7 100644
--- a/.github/workflows/adhoctest.yml
+++ b/.github/workflows/adhoctest.yml
@@ -25,6 +25,9 @@ on:
     paths:
       - '**/workflows/adhoctest.yml'
 
+permissions:
+  contents: read
+
 jobs:
   build:
 
diff --git a/src/docker/Dockerfile b/src/docker/Dockerfile
index 943a1c612..eb5c9ad54 100644
--- a/src/docker/Dockerfile
+++ b/src/docker/Dockerfile
@@ -17,7 +17,7 @@
 # for Linux 64-bit and 32-bit, and Windows 64-bit and 32-bit.
 # See the README for further details.
 
-FROM ubuntu:18.04
+FROM ubuntu:18.04@sha256:152dc042452c496007f07ca9127571cb9c29697f42acbfad72324b2bb2e43c98
 
 ENV JAVA_HOME=/usr/lib/jvm/java-8-openjdk-amd64
 ENV MAVEN_HOME=/opt/maven