Explicitely pull base image before build to workaround a Travis bug

This should avoid those weird and inconsistent (but frequent: 80% of builds!) error:

```
alpinelab#8 [internal] load metadata for docker.io/library/ruby:3.0.3-alpine

alpinelab#8 sha256:f4fb592f325c2dc174e0e72aea227cd1033c354a69e26a749fec5f91626b4554

alpinelab#8 ERROR: failed to copy: httpReadSeeker: failed open: content at https://mirror.gcr.io/v2/library/ruby/manifests/sha256:8ec4194e3df279ef459ed06724dd455964defcf49857c161154378a0e4df98a9?ns=docker.io not found: not found

------

 > [internal] load metadata for docker.io/library/ruby:3.0.3-alpine:

------

failed to solve with frontend dockerfile.v0: failed to solve with frontend gateway.v0: rpc error: code = Unknown desc = failed to copy: httpReadSeeker: failed open: content at https://mirror.gcr.io/v2/library/ruby/manifests/sha256:8ec4194e3df279ef459ed06724dd455964defcf49857c161154378a0e4df98a9?ns=docker.io not found: not found
```

Author: michaelbaudino

build.sh

@@ -7,6 +7,8 @@ fi
 
 set -ev
 
+docker pull ruby:${RUBY_VERSION} # (needlessly mandatory to prevent a bug in Travis)
+
 docker build . \
   --build-arg BASE_IMAGE_TAG=${RUBY_VERSION} \
   --tag alpinelab/ruby-dev:${RUBY_VERSION} \