diff --git a/Dockerfile b/Dockerfile
index ccf465f..67de752 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -77,14 +77,14 @@ RUN set -e; \
       mkdir -p /src && cd /src; \
       cp "${LOCAL_TARBALL}" exchange.tar.gz; \
       tar xzf exchange.tar.gz; \
-      EXCHANGE_DIR="$(ls -d taler-exchange-* exchange-* 2>/dev/null | head -1)"; \
+      EXCHANGE_DIR="$(tar -tzf exchange.tar.gz | head -1 | cut -d/ -f1)"; \
       EXCHANGE_TAR=1; \
     elif [ -n "${EXCHANGE_TARBALL_URL}" ]; then \
       echo "Fetching Exchange tarball ${EXCHANGE_TARBALL_URL}"; \
       mkdir -p /src && cd /src; \
       curl -L "${EXCHANGE_TARBALL_URL}" -o exchange.tar.gz; \
       tar xzf exchange.tar.gz; \
-      EXCHANGE_DIR="$(ls -d taler-exchange-* exchange-* 2>/dev/null | head -1)"; \
+      EXCHANGE_DIR="$(tar -tzf exchange.tar.gz | head -1 | cut -d/ -f1)"; \
       EXCHANGE_TAR=1; \
     else \
       git clone --depth 1 --branch "${EXCHANGE_REF}" https://git.taler.net/exchange.git /src/exchange; \
@@ -123,14 +123,14 @@ RUN set -e; \
       mkdir -p /src && cd /src; \
       cp "${LOCAL_TARBALL}" merchant.tar.gz; \
       tar xzf merchant.tar.gz; \
-      MERCHANT_DIR="$(ls -d taler-merchant-* merchant-* 2>/dev/null | head -1)"; \
+      MERCHANT_DIR="$(tar -tzf merchant.tar.gz | head -1 | cut -d/ -f1)"; \
       MERCHANT_TAR=1; \
     elif [ -n "${MERCHANT_TARBALL_URL}" ]; then \
       echo "Fetching Merchant tarball ${MERCHANT_TARBALL_URL}"; \
       mkdir -p /src && cd /src; \
       curl -L "${MERCHANT_TARBALL_URL}" -o merchant.tar.gz; \
       tar xzf merchant.tar.gz; \
-      MERCHANT_DIR="$(ls -d merchant-* | head -1)"; \
+      MERCHANT_DIR="$(tar -tzf merchant.tar.gz | head -1 | cut -d/ -f1)"; \
       MERCHANT_TAR=1; \
     else \
       git clone --depth 1 --branch "${MERCHANT_REF}" https://git.taler.net/merchant.git /src/merchant; \