Multi-stage Docker builds (#2850)

This builds on @S7evinK's work to make multi-stage Docker builds. Now
that we can build SQLite without Cgo this should be much simpler and
should make Docker builds in CI significantly faster.

Co-authored-by: Till Faelligen <tfaelligen@gmail.com>
Co-authored-by: Till Faelligen <davidf@element.io>
Co-authored-by: Till Faelligen <2353100+S7evinK@users.noreply.github.com>

.github/workflows/dendrite.yml

@@ -297,7 +297,7 @@ jobs:
         SYTEST_BRANCH: ${{ github.head_ref }}
         CGO_ENABLED: ${{ matrix.cgo && 1 }}
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v3
       - name: Run Sytest
         run: /bootstrap.sh dendrite
         working-directory: /src
@@ -364,8 +364,8 @@ jobs:
           sudo apt-get update && sudo apt-get install -y libolm3 libolm-dev
           go get -v github.com/gotesttools/gotestfmt/v2/cmd/gotestfmt@latest
 
-      - name: Run actions/checkout@v2 for dendrite
-        uses: actions/checkout@v2
+      - name: Run actions/checkout@v3 for dendrite
+        uses: actions/checkout@v3
         with:
           path: dendrite