Oh actually install the apt packages in docker, impressed it gets this far

.travis.yml

@@ -18,13 +18,20 @@ env:
     - SDK_URL=https://bitcoincore.org/depends-sources/sdks
     - DOCKER_PACKAGES="build-essential gcc-6 g++-6 cmake git libcap-dev bsdmainutils curl git ca-certificates ccache"
   matrix:
-      - HOST=x86_64-unknown-linux-gnu PACKAGES="gperf cmake"
+      - HOST=x86_64-unknown-linux-gnu
+install:
+    - env | grep -E '^(CCACHE_|DISPLAY|CONFIG_SHELL)' | tee /tmp/env
+    - if [[ $HOST = *-mingw32 ]]; then DOCKER_ADMIN="--cap-add SYS_ADMIN"; fi
+    - DOCKER_ID=$(docker run $DOCKER_ADMIN -idt --mount type=bind,src=$TRAVIS_BUILD_DIR,dst=$TRAVIS_BUILD_DIR --mount type=bind,src=$CCACHE_DIR,dst=$CCACHE_DIR -w $TRAVIS_BUILD_DIR --env-file /tmp/env ubuntu:18.04)
+    - DOCKER_EXEC="docker exec $DOCKER_ID"
+    - if [ -n "$DPKG_ADD_ARCH" ]; then $DOCKER_EXEC dpkg --add-architecture "$DPKG_ADD_ARCH" ; fi
+    - travis_retry $DOCKER_EXEC apt-get update
+    - travis_retry $DOCKER_EXEC apt-get install --no-install-recommends --no-upgrade -qq $PACKAGES $DOCKER_PACKAGES
 script:
   - export TRAVIS_COMMIT_LOG=`git log --format=fuller -1`
   - OUTDIR=$BASE_OUTDIR/$TRAVIS_PULL_REQUEST/$TRAVIS_JOB_NUMBER-$HOST
   - if [ -z "$NO_DEPENDS" ]; then $DOCKER_EXEC ccache --max-size=$CCACHE_SIZE; fi
   - $DOCKER_EXEC bash -c "update-alternatives --install /usr/bin/gcc gcc /usr/bin/gcc-6 60 --slave /usr/bin/g++ g++ /usr/bin/g++-6"
-  - $DOCKER_EXEC bash -c "gcc -v; g++ -v"
   - $DOCKER_EXEC bash -c "mkdir build && cd build && cmake .. && make $MAKEJOBS"
 after_script:
   - echo $TRAVIS_COMMIT_RANGE