Disable pip cache in Dockerfile

[strayer]

It doesn't do much of a difference, but still saves 1MB in the pip install layer.

Without --no-cache-dir:

IMAGE               CREATED             CREATED BY                                      SIZE                COMMENT
4a889526f024        23 seconds ago      /bin/sh -c #(nop)  ENTRYPOINT ["ouroboros"]     0B
fc1bbdc7d8d9        23 seconds ago      /bin/sh -c pip install .                        11.2MB
f52b1822a574        29 seconds ago      /bin/sh -c #(nop) WORKDIR /app                  0B
ff68c7715a92        29 seconds ago      /bin/sh -c #(nop) COPY dir:b7a988c2253999fea…   21.4kB

With --no-cache-dir:

IMAGE               CREATED             CREATED BY                                      SIZE                COMMENT
fc05a396dea7        4 seconds ago       /bin/sh -c #(nop)  ENTRYPOINT ["ouroboros"]     0B
4cfeeb1816ee        4 seconds ago       /bin/sh -c pip install --no-cache-dir .         10.2MB
1d752126f43d        10 seconds ago      /bin/sh -c #(nop) WORKDIR /app                  0B
8ea197c11b15        11 seconds ago      /bin/sh -c #(nop) COPY dir:8a8a1d6ff52947334…   21.4kB

[codecov-io]

Codecov Report

Merging #50 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff          @@
##           master    #50   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           7      7           
  Lines         149    149           
=====================================
  Hits          149    149

---

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[circa10a]

Hey 1mb is 1mb!

Thanks!