Dockerfile: Fix missing gnulib dependency
Dockerfile: Fix missing gnulib dependency

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

Signed-off-by: Julien Hachenberger <af7aa258454f8780be83de4cc028f818c8dfdb7c@sit.fraunhofer.de>

tpm2-software/tpm2-tss,01org/TPM2.0-TSS,tpm2-software/tpm2-tss,01org/TPM2.0-TSS,01org/TPM2.0-TSS,01org/tpm2-tss,01org/tpm2-tss,01org/tpm2-tss,tpm2-software/tpm2-tss,01org/TPM2.0-TSS