hotfix in Dockerfile to have idemixgen under /tmp/bin (hyperledger#73)

Signed-off-by: pco <pasquale.convertini@ibm.com>

Author: pasquale95

Dockerfile

@@ -31,7 +31,7 @@ RUN go build -o /tmp/bin/configtxgen ./tools/configtxgen
 RUN go build -o /tmp/bin/cryptogen ./tools/cryptogen
 RUN go build -o /tmp/bin/configtxlator ./tools/configtxlator
 RUN go build -o /tmp/bin/fxconfig ./tools/fxconfig
-RUN go install github.com/IBM/idemix/tools/idemixgen@$IDEMIX_VERSION
+RUN GOBIN=/tmp/bin go install github.com/IBM/idemix/tools/idemixgen@$IDEMIX_VERSION
 
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 # Stage 2: Production runtime image