build-system: Docker build for 64bit MXE

We previously tried to build the MXE Docker container on GitHub using
an Action, but that really didn't work well and was a lot more trouble
than it was worth.

So this goes back to an offline build mechanism where I simply create
an updated Docker image when needed and push that to Docker Hub.

But this nearly hides the most interesting change here - we are finally
switching to using 64bit binaries on Windows. It's 2020 and fewer than
1% of our users use 32bit Windows machines. We'll need to expand this
to be able to have both a 32bit and a 64bit version of Subsurface for
Windows. But for now, this solves the problem for 99% of our users.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>

.github/workflows/mxe-dockerimage-stage1.yml

@@ -1,53 +0,0 @@
-name: MXE Docker Image CI
-
-on:
-  push:
-    paths:
-    - scripts/docker/mxe-build-container/*
-    - .github/workflows/mxe-docker*
-
-jobs:
-  mxe-build-container-stage1:
-    runs-on: ubuntu-latest
-    env:
-      # IMPORTANT: use the second digit to test new versions, each new official build should be n.0 with the first one being 1.0
-      #            as you test changes toward a new release, call those 1.1, 1.2, 1.3, etc
-      #            VERSION is just that version presented as a string constant
-      #
-      VERSION: ${{ '1.0' }}
-
-    steps:
-    - uses: actions/checkout@v1
-
-      # Because, reasons, we can't really do anything in YAML, so do this crazy shell callout thingy in order to assemble
-      # sane variables to use later in this job - if someone knows an easier way to do this, please let me know!
-      # This abomination below assembles the docker image NAME and appends ".stage1" to the VERSION
-    - name: set env
-      run: |
-        v=${{ env.VERSION }}
-        s=".stage1"
-        b=${{ github.ref }} # -BRANCH suffix, unless the branch is master
-        b=${b/refs\/heads\//}
-        b=${b,,} # the name needs to be all lower case
-        if [ $b = "master" ] ; then b="" ; else b="-$b" ; fi
-        echo "::set-env name=NAME::subsurface/mxe-build-container${b}:${v}${s}"
-
-    - name: Build and Publish stage 1 Docker image to Dockerhub
-      uses: elgohr/Publish-Docker-Github-Action@master
-      with:
-        name:       ${{ env.NAME }}
-        username:   ${{ secrets.DOCKER_USERNAME }}
-        password:   ${{ secrets.DOCKER_PASSWORD }}
-        dockerfile: 'Dockerfile-stage1'
-        workdir:    './scripts/docker/mxe-build-container/'
-
-    - name: Trigger stage 2 to run
-      # this triggers a dispatch event in our own repository - this allows us to work around the
-      # 6h runtime max (as on a two core system the total build time of this container is closer to 7h)
-      # we use the event_type to pass the version that we are building to the second stage
-      run: |
-        curl -XPOST -H 'authorization: token ${{ secrets.ACCESS_TOKEN }}' \
-                    -H "Accept: application/vnd.github.everest-preview+json" \
-                    -H "Content-Type: application/json" \
-                    https://api.github.com/repos/subsurface-divelog/subsurface/dispatches \
-                    --data '{"event_type": "${{ env.VERSION }}", "client_payload": { "target_branch": "${{ github.ref }}" } }'

.github/workflows/mxe-dockerimage-stage2.yml

@@ -1,40 +0,0 @@
-name: MXE stage 2 Docker Image CI
-
-# trigger this second stage via a repository dispaych event
-# the version that we are building is passed in via the event type that is available to us here
-# as github.event.action
-
-on: repository_dispatch
-
-jobs:
-  mxe-build-container-stage2:
-    runs-on: ubuntu-latest
-
-    steps:
-    - uses: actions/checkout@v1
-      with:
-        ref: ${{ github.event.client_payload.target_branch }}
-
-      # Grab the version from the event name that we were triggered by and add ".stage1" to find the docker image to start FROM
-      # And create the NAME of the final docker image (including the branch if not master)
-    - name: set env
-      run: |
-        v=${{ github.event.action }}
-        s=".stage1"
-        b=${{ github.event.client_payload.target_branch }} # -BRANCH suffix, unless the branch is master
-        b=${b/refs\/heads\//}
-        b=${b,,} # the name needs to be all lower case
-        if [ "$b" = "master" ] || [ "$b" = "" ] ; then b="" ; else b="-$b" ; fi
-        echo "::set-env name=VERSION::${v}${s}"
-        echo "::set-env name=NAME::subsurface/mxe-build-container${b}:${v}"
-
-    - name: Build and Publish stage 2 Docker image to Dockerhub
-      uses: elgohr/Publish-Docker-Github-Action@master
-      with:
-        name:       ${{ env.NAME }}
-        username:   ${{ secrets.DOCKER_USERNAME }}
-        password:   ${{ secrets.DOCKER_PASSWORD }}
-        dockerfile: 'Dockerfile-stage2'
-        workdir:    './scripts/docker/mxe-build-container/'
-        buildargs:  VERSION
-