re-order custom apt-packages before pip requirements (b/163169552)

[ajslone]

This PR changes the docker ordering to add custom apt packages before installing python pip dependencies.

[codecov]

Codecov Report

Merging #82 into master will decrease coverage by 0.03%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master      #82      +/-   ##
==========================================
- Coverage   51.80%   51.77%   -0.04%     
==========================================
  Files          31       31              
  Lines        3154     3154              
==========================================
- Hits         1634     1633       -1     
- Misses       1520     1521       +1     

Impacted Files	Coverage Δ
caliban/docker/build.py	33.17% <0.00%> (ø)
caliban/util/auth.py	66.66% <0.00%> (-9.53%)	⬇️
caliban/platform/gke/util.py	71.37% <0.00%> (+0.38%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 2b4cb13...8826813. Read the comment docs.

[sritchie]

Looks great!