ref(Dockerfiles): use ubuntu-debootstrap:14.04 as base image

The difference in virtual sizes on my boot2docker builds, YMMV:
```console
$ docker images
REPOSITORY           TAG              IMAGE ID         CREATED              VIRTUAL SIZE
deis/store-gateway   ubuntu           255a52a16581     34 seconds ago       485.7 MB
deis/store-gateway   debootstrap      8be7773d3684     3 minutes ago        389.9 MB
deis/store-monitor   ubuntu           92889ab5899b     About a minute ago   440.3 MB
deis/store-monitor   debootstrap      089b398bc018     3 minutes ago        322.9 MB
deis/store-daemon    ubuntu           93634a31d925     About a minute ago   440.3 MB
deis/store-daemon    debootstrap      eeab0dbf720b     3 minutes ago        322.9 MB
deis/store-base      ubuntu           aff69a86d13e     About a minute ago   440.3 MB
deis/store-base      debootstrap      2c9e6b01b297     3 minutes ago        322.9 MB
deis/router          ubuntu           14557e2086bf     About a minute ago   254.8 MB
deis/router          debootstrap      c4a1a996f92b     4 minutes ago        155.6 MB
deis/publisher       ubuntu           545c3e85793f     2 minutes ago        5.032 MB
deis/publisher       debootstrap      339f13ab5bee     4 minutes ago        5.032 MB
deis/logspout        ubuntu           f26b540224cd     2 minutes ago        12.29 MB
deis/logspout        debootstrap      e3abf90eb770     4 minutes ago        12.29 MB
deis/logger          ubuntu           5f6718986d35     3 minutes ago        4.782 MB
deis/logger          debootstrap      a202ec92bb8e     4 minutes ago        4.782 MB
deis/registry        ubuntu           db8f747c93ba     2 minutes ago        499.9 MB
deis/registry        debootstrap      85982cd2b12a     19 minutes ago       416.1 MB
deis/database        ubuntu           110665976fab     3 minutes ago        535.5 MB
deis/database        debootstrap      6a4eeff98b7a     22 minutes ago       480.1 MB
deis/controller      ubuntu           9c7ce6599741     3 minutes ago        342 MB
deis/controller      debootstrap      926abfd02d6c     25 minutes ago       259.3 MB
deis/cache           ubuntu           8565f218ce4f     5 minutes ago        256 MB
deis/cache           debootstrap      56b33815fee4     27 minutes ago       133.7 MB
deis/builder         ubuntu           8e487a1e09f3     5 minutes ago        1.487 GB
deis/builder         debootstrap      7e04cf6a4516     28 minutes ago       1.428 GB
ubuntu               14.04            9cbaf023786c     3 days ago           192.8 MB
ubuntu-debootstrap   14.04            f4bf82423f0c     3 days ago           87.16 MB
```

ping @aledbf!