build: simple modification to allow docker develop builds on aarch64 (#5475)

## Issue being fixed or feature implemented
Building with develop docker container on aarch64

## What was done?
Only install i386 stuff on non-arm builders

## How Has This Been Tested?
Building on aarch64 / m1 

## Breaking Changes
Should be none

## Checklist:
_Go over all the following points, and put an `x` in all the boxes that
apply._
- [ ] I have performed a self-review of my own code
- [ ] I have commented my code, particularly in hard-to-understand areas
- [ ] I have added or updated relevant unit/integration/functional/e2e
tests
- [ ] I have made corresponding changes to the documentation
- [ ] I have assigned this pull request to a milestone _(for repository
code-owners and collaborators only)_

contrib/containers/ci/Dockerfile

@@ -6,7 +6,14 @@ ENV DEBIAN_FRONTEND="noninteractive" TZ="Europe/London"
 # Build and base stuff
 # (zlib1g-dev is needed for the Qt host binary builds, but should not be used by target binaries)
 ENV APT_ARGS="-y --no-install-recommends --no-upgrade"
-RUN dpkg --add-architecture i386
+
+
+# Install packages for i386; disabled on aarch64 and arm64 hosts
+RUN (dpkg --print-architecture | grep -Eq 'aarch64|arm64' || dpkg --add-architecture i386)
+RUN (dpkg --print-architecture | grep -Eq 'aarch64|arm64' || (apt-get update && apt-get install $APT_ARGS \
+    g++-9-multilib \
+    wine32) && rm -rf /var/lib/apt/lists/*)
+
 RUN apt-get update && apt-get install $APT_ARGS \
     autotools-dev \
     automake \
@@ -61,7 +68,6 @@ RUN useradd -u ${USER_ID} -g dash -s /bin/bash -m -d /home/dash dash
 RUN apt-get update && apt-get install $APT_ARGS \
     bc \
     gawk \
-    g++-9-multilib \
     g++-arm-linux-gnueabihf \
     g++-mingw-w64-x86-64 \
     jq \
@@ -74,7 +80,6 @@ RUN apt-get update && apt-get install $APT_ARGS \
     parallel \
     valgrind \
     wine-stable \
-    wine32 \
     wine64 \
     xorriso