Use busybox in fixtures, instead of ubuntu

Signed-off-by: d11wtq <chris@w3style.co.uk>

tests/fixtures/longer-filename-figfile/fig.yaml

@@ -1,3 +1,3 @@
 definedinyamlnotyml:
-  image: ubuntu
+  image: busybox:latest
   command: /bin/sleep 300

tests/fixtures/multiple-figfiles/fig.yml

@@ -1,6 +1,6 @@
 simple:
-  image: ubuntu
+  image: busybox:latest
   command: /bin/sleep 300
 another:
-  image: ubuntu
+  image: busybox:latest
   command: /bin/sleep 300

tests/fixtures/multiple-figfiles/fig2.yml

@@ -1,3 +1,3 @@
 yetanother:
-  image: ubuntu
+  image: busybox:latest
   command: /bin/sleep 300

tests/fixtures/simple-dockerfile/Dockerfile

@@ -1,2 +1,2 @@
-FROM ubuntu
+FROM busybox:latest
 CMD echo "success"

tests/fixtures/simple-figfile/fig.yml

@@ -1,6 +1,6 @@
 simple:
-  image: ubuntu
+  image: busybox:latest
   command: /bin/sleep 300
 another:
-  image: ubuntu
+  image: busybox:latest
   command: /bin/sleep 300