Change working directory for Docker build

An attempt to use learnings from here:

actions/runner#1009

.github/workflows/gcc-problems.yml

@@ -100,7 +100,7 @@ jobs:
         with:
           image: magma/mme_builder:latest
           # TODO: Break up this long line (can I use '\' at end of each line?)
-          run: cd /magma/lte/gateway;make build_oai 2>&1 > /compile.log;for file in ${{ steps.changed_files.outputs.all }}; do grep $file /compile.log | xo '/\/magma\/((.*):(\d+):(\d+):\s+(?:fatal\s)?(warning|error):\s+(.*))/$1/' || true; done;
+          run: cd lte/gateway;make build_oai 2>&1 > /compile.log;for file in ${{ steps.changed_files.outputs.all }}; do grep $file /compile.log | xo '/\/magma\/((.*):(\d+):(\d+):\s+(?:fatal\s)?(warning|error):\s+(.*))/$1/' || true; done;
       -
         name: Store oai_build_logs artifact
         uses: actions/upload-artifact@v2