Dockerfile: Fix missing gnulib dependency

Since #1180 the call to `./bootstrap` requires gnulib. (closes #1215)

Signed-off-by: Julien Hachenberger <julien.hachenberger@sit.fraunhofer.de>

diff --git a/Dockerfile b/Dockerfile
index 6bea523..b3d44e9 100644
--- a/Dockerfile
+++ b/Dockerfile

@@ -7,6 +7,7 @@
     g++ \
     gcc \
     git \
+    gnulib \
     libssl-dev \
     libtool \
     m4 \
@@ -49,7 +50,7 @@
 # TPM2-TSS
 COPY . /tmp/tpm2-tss/
 WORKDIR /tmp/tpm2-tss
-RUN ./bootstrap \
+RUN ./bootstrap -I /usr/share/gnulib/m4 \
 	&& ./configure --enable-unit \
 	&& make -j$(nproc) check \
 	&& make install \