install gcc6 after some basics

.travis.yml

@@ -16,7 +16,7 @@ env:
     - CCACHE_DIR=$HOME/.ccache
     - BASE_OUTDIR=$TRAVIS_BUILD_DIR/out
     - 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"
+    - DOCKER_PACKAGES="build-essential cmake git libcap-dev bsdmainutils curl git ca-certificates ccache"
   matrix:
       - HOST=x86_64-w64-mingw32
       - HOST=x86_64-unknown-linux-gnu
@@ -27,9 +27,11 @@ install:
     - 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:16.04)
     - 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 software-properties-common -qq $PACKAGES $DOCKER_PACKAGES
     - travis_retry $DOCKER_EXEC add-apt-repository ppa:ubuntu-toolchain-r/test
     - travis_retry $DOCKER_EXEC apt-get update
-    - travis_retry $DOCKER_EXEC apt-get install --no-install-recommends --no-upgrade -qq $PACKAGES $DOCKER_PACKAGES
+    - travis_retry $DOCKER_EXEC apt-get install --no-install-recommends --no-upgrade -qq gcc-6 g++-6
 script:
   - export TRAVIS_COMMIT_LOG=`git log --format=fuller -1`
   - OUTDIR=$BASE_OUTDIR/$TRAVIS_PULL_REQUEST/$TRAVIS_JOB_NUMBER-$HOST