diff --git a/CHANGELOG.md b/CHANGELOG.md
index 1f06d22..24d0f33 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -8,6 +8,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### Fixed
 - Fix buildbot version in upgraded platforms ([#32](../../pull/32))
 
+### Changed
+- Optimize build cache for PR builds ([#36](../../pull/36))
+
 ## [2.4.1-1] - 2019-09-20
 ### Added
 - Upgrade to buildbot 2.4.1 ([#29](../../pull/29))
diff --git a/scripts/travis.sh b/scripts/travis.sh
index ef980c0..574f78b 100755
--- a/scripts/travis.sh
+++ b/scripts/travis.sh
@@ -5,7 +5,14 @@
 # When a tag is built (TRAVIS_TAG), the branch cannot be determined from
 # TRAVIS_BRANCH. Assume that it is a release tag: If the version is 1.8.x, we're
 # on the 1.8 maintenance branch. Otherwise, we're on master.
-if [ -z "$TRAVIS_TAG" ]
+#
+# For PR builds (TRAVIS_PULL_REQUEST_BRANCH), use the branch from which the PR
+# originated, not the target branch. This means that the same image is used for
+# the Docker build cache as for the corresponding branch build.
+if [ -n "$TRAVIS_PULL_REQUEST_BRANCH" ]
+then
+    BRANCH="$TRAVIS_PULL_REQUEST_BRANCH"
+elif [ -z "$TRAVIS_TAG" ]
 then
     BRANCH="$TRAVIS_BRANCH"
 elif [[ "$TRAVIS_TAG" =~ ^v1\.8\. ]]