Merge JoinMarket-Org/joinmarket-clientserver#1590: fix Dockerfile gpg err

30d971504d Update Dockerfile (st3b1t)

Pull request description:

  solve this at build:
  ```
  GPG not installed, cannot verify release signatures; install gpg or use --no-gpg-validation.
  Terminated
  ```

ACKs for top commit:
  kristapsk:
    utACK 30d971504d
  roshii:
    utACK 30d971504d

Tree-SHA512: 94696861a11175ebac728afecb3923a39d682fc58f6a391ff193b37f62645c0fa39b3cc56f9b5a48cbd13deca18aaa64620a1d11d2fac7c02d7a6c4d5fbfc05a

Dockerfile

@@ -5,7 +5,7 @@ WORKDIR /jm/clientserver
 
 COPY . .
 
-RUN apt-get update && apt-get install -y --no-install-recommends ca-certificates=* curl=* \
+RUN apt-get update && apt-get install -y --no-install-recommends gnupg ca-certificates=* curl=* \
   python3-pip=* python3=* \
   && pip3 install 'wheel>=0.35.1' \
   && ./install.sh --docker-install \