add hadolint to travisfile

Signed-off-by: Felix Bartels <felix@host-consultants.de>

.travis.yml

@@ -1,11 +1,20 @@
 dist: xenial
 
+env:
+  global:
+    - HADOLINT_VERSION=1.17.1
+
 language: rust
 rust: nightly
 cache: cargo
 
+before_install:
+  - sudo curl -L https://github.com/hadolint/hadolint/releases/download/v$HADOLINT_VERSION/hadolint-$(uname -s)-$(uname -m) -o /usr/local/bin/hadolint
+  - sudo chmod +rx /usr/local/bin/hadolint
+
 # Nothing to install
 install: true
 script:
+- git ls-files --exclude='Dockerfile*' --ignored | xargs --max-lines=1 hadolint
 - cargo build --features "sqlite"
 - cargo build --features "mysql"