Downcase container image name to avoid invalid format

[dkapanidis]

When the directory where Dockerfile resides contains upper-case characters and they are transformed to image name it is an invalid repository name.

To avoid this when inferring settings the directory name should be downcased to map it to a valid repository name.

Example below:

➜  git clone https://github.com/Runnable/dockerfiles
Cloning into 'dockerfiles'...
remote: Counting objects: 1092, done.
remote: Total 1092 (delta 0), reused 0 (delta 0), pack-reused 1092
Receiving objects: 100% (1092/1092), 9.33 MiB | 1.28 MiB/s, done.
Resolving deltas: 100% (296/296), done.
Checking connectivity... done.
➜  cd dockerfiles
➜  captain build
[CAPTAIN] No configuration found [captain.json captain.yaml captain.yml] - inferring values
[CAPTAIN] Building image spiddy/UTlPL1-f2W1TAAAW:latest
API error (500): Invalid repository name (UTlPL1-f2W1TAAAW), only [a-z0-9-_.] are allowed