ci: Workaround for BuildKit ARG substitution bug

Docker BuildKit does not seem to support shell substitution on ARG
variables, either as a bug or as an undocumented behaviour.

For more details, refer to the issue moby/buildkit#5156.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>

Author: stephanosio

Dockerfile.ci

@@ -62,7 +62,8 @@ EOF
 # NOTE: Pre-built Doxygen binaries are only available for x86_64 host.
 RUN <<EOF
 	if [ "${HOSTTYPE}" = "x86_64" ]; then
-		wget ${WGET_ARGS} "https://github.com/doxygen/doxygen/releases/download/Release_${DOXYGEN_VERSION//./_}/doxygen-${DOXYGEN_VERSION}.linux.bin.tar.gz"
+		DOXYGEN_DIR="$(echo "${DOXYGEN_VERSION}" | sed 's/./_/g')"
+		wget ${WGET_ARGS} "https://github.com/doxygen/doxygen/releases/download/Release_${DOXYGEN_DIR}/doxygen-${DOXYGEN_VERSION}.linux.bin.tar.gz"
 		tar xf doxygen-${DOXYGEN_VERSION}.linux.bin.tar.gz -C /opt
 		ln -s /opt/doxygen-${DOXYGEN_VERSION}/bin/doxygen /usr/local/bin
 		rm doxygen-${DOXYGEN_VERSION}.linux.bin.tar.gz