Add arm64 support to Docker image + add librocksdb.a for linux aarch64 (#84)

This commit:

- Adds building docker images for multiple architectures (currently intel, arm64) 
  using docker `buildx`
- Temporarily modifies the `Dockerfile` to use `ppa:mainnet-pat/opt-qt-5.13.2-bionic` 
  which is a clone of previously used `ppa:beineri/opt-qt-5.13.2-bionic` but builds 
  arm64 deb packages as well. Author is contacted to support this in their repo.
    - Calin verified that the input source tree and build control files are identical to ~beineri
- Adds librocksdb.a static lib built for arm64 (should be generally useful outside of docker)

.travis.yml

@@ -6,6 +6,11 @@ jobs:
   fast_finish: true
   include:
     - os: linux
+      arch: amd64
+      dist: bionic
+      env: TARGET_OS=ubuntu
+    - os: linux
+      arch: arm64
       dist: bionic
       env: TARGET_OS=ubuntu
     - os: osx

contrib/docker/Dockerfile

@@ -1,11 +1,11 @@
-FROM ubuntu:bionic
+FROM --platform=$BUILDPLATFORM ubuntu:bionic
 LABEL maintainer="Axel Gembe <derago@gmail.com>"
 
 ARG MAKEFLAGS
 
 RUN apt-get update -y && \
     apt-get install -y software-properties-common && \
-    add-apt-repository ppa:beineri/opt-qt-5.13.2-bionic && \
+    add-apt-repository ppa:mainnet-pat/opt-qt-5.13.2-bionic && \
     apt-get update -y && \
     apt-get install -y qt513base openssl && \
     apt-get install -y git build-essential pkg-config zlib1g-dev libbz2-dev libjemalloc-dev libjemalloc1 libzmq3-dev && \

contrib/docker/build.sh

@@ -12,4 +12,5 @@ IMAGE_TAG="$1"
 here=$(dirname $(realpath "$0" 2> /dev/null || grealpath "$0"))
 . "$here"/../base.sh
 
-docker build --build-arg MAKEFLAGS="-j $WORKER_COUNT" -t "$IMAGE_TAG" -f contrib/docker/Dockerfile "$here"/../..
+docker buildx build --build-arg MAKEFLAGS="-j $WORKER_COUNT" -t "$IMAGE_TAG" -f contrib/docker/Dockerfile \
+    --platform linux/arm64/v8,linux/amd64 --push "$here"/../..

contrib/travis/linux.ubuntu.before_install

@@ -1,3 +1,3 @@
 #!/bin/sh
-sudo add-apt-repository -y ppa:beineri/opt-qt-5.13.2-bionic
+sudo add-apt-repository -y ppa:mainnet-pat/opt-qt-5.13.2-bionic
 sudo apt-get update -qq