Ensure apt is up-to-date before installing stuff

Without this, I see the following error when trying to build the container:

```
WARNING: The following packages cannot be authenticated!
  libroken18-heimdal libasn1-8-heimdal libgpg-error0 libgcrypt11 libp11-kit0
  libtasn1-3 libgnutls26 libkrb5support0 libk5crypto3 libkeyutils1 libkrb5-3
  libgssapi-krb5-2 libidn11 libhcrypto4-heimdal libheimbase1-heimdal
  libwind0-heimdal libhx509-5-heimdal libkrb5-26-heimdal libheimntlm0-heimdal
  libgssapi3-heimdal libsasl2-2 libldap-2.4-2 librtmp0 openssl ca-certificates
  libcurl3-gnutls iso-codes krb5-locales libsasl2-modules python-apt-common
  python-apt python-gnupginterface xz-lzma unattended-upgrades python-pycurl
  python-software-properties
E
: 
There are problems and -y was used without --force-yes


Error: The command [/bin/sh -c apt-get install -y python-software-properties] returned a non-zero code: 100
```