Merge #13998: Scripts and tools: gitian-build.py improvements and corrections

0f22a0cf2f Fix gitian-build.py --verify option (Hennadii Stepanov)
4c56a798c0 Set/unset USE_LXC, USE_VBOX, USE_DOCKER explicitly (Hennadii Stepanov)
cbbd98863b Fix Docker related issues for gitian-build.py (Hennadii Stepanov)

Pull request description:

  1. The Docker does not depend on `apt-cacher-ng` package. Ref: #14002.

  2. Do not try to install the Docker if `docker.service` is detected on the system (e.g., the Docker was installed manually). Fix https://github.com/bitcoin/bitcoin/pull/13623#issuecomment-405684241 by **Sjors**.

  3. Prevent the setting of more than one environment variable for the `gitian-builder` (an alternative to #13999). E.g., USE_LXC being set shadows USE_DOCKER; for details see [`gitian-builder/libexec/make-clean-vm`](https://github.com/devrandom/gitian-builder/blob/93a62c7d7d018c66c02a19bac3d751144043cfec/libexec/make-clean-vm#L7):
  ```sh
  VMSW=KVM
  if [ -n "$USE_LXC" ]; then
      VMSW=LXC
  elif [ -n "$USE_VBOX" ]; then
      VMSW=VBOX
  elif [ -n "$USE_DOCKER" ]; then
      VMSW=DOCKER
  fi
  ```
  4. The [`gitian-builder/bin/gverify`](https://github.com/devrandom/gitian-builder/blob/master/bin/gverify) script returns the exit code 1 if a signature verification ends with 'BAD SIGNATURE' or 'MISMATCH' by design. This PR allows to see the verification results for all signatures without a premature fail of the `gitian-build.py` script. Ref: #14014.

ACKs for commit 0f22a0:

Tree-SHA512: 55f8a5cffa20d0c745f51a687f3199cea015fa616e56a0aee4c25b5ca0985036c61e8cf1922515338d8c6a85f873674ebe7a9a56a5069d65a187e383150f1a83

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