fix: gracefully skip pinned images

[piksel]

This PR adds a check when pulling images to see whether it's a pinned image hash, and skips the image with a message identifying the problem.

Before:

INFO[0001] Running a one time update.
WARN[0002] Could not do a head request for "sha256:fa5269854a5e615e51a72b17ad3fd1e01268f278a6684c8ed3c5f0cdce3f230b", falling back to regular pull.  container=/shatest1 image="sha256:fa5269854a5e615e51a72b17ad3fd1e01268f278a6684c8ed3c5f0cdce3f230b"
WARN[0002] Reason: registry responded to head request with "401 Unauthorized", auth: "Bearer realm=\"https://auth.docker.io/token\",service=\"registry.docker.io\",scope=\"repository:library/sha256:pull\",error=\"insufficient_scope\""  container=/shatest1 image="sha256:fa5269854a5e615e51a72b17ad3fd1e01268f278a6684c8ed3c5f0cdce3f230b"
INFO[0003] Unable to update container "/shatest1": Error response from daemon: pull access denied for sha256, repository does not exist or may require 'docker login': denied: requested access to the resource is denied. Proceeding to next.
INFO[0003] Session done                                  Failed=0 Scanned=1 Updated=0 notify=no
INFO[0003] Waiting for the notification goroutine to finish  notify=no

After:

INFO[0001] Running a one time update.
INFO[0001] Unable to update container "/shatest1": container uses a pinned image, and cannot be updated by watchtower. Proceeding to next.
INFO[0001] Session done                                  Failed=0 Scanned=1 Updated=0 notify=no
INFO[0001] Waiting for the notification goroutine to finish  notify=no

Additionally, this puts all the options for the docker client wrapper in a struct for increased legibility.

[codecov]

Codecov Report

Merging #1277 (20806d4) into main (56368a7) will increase coverage by 0.51%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##             main    #1277      +/-   ##
==========================================
+ Coverage   63.13%   63.65%   +0.51%     
==========================================
  Files          23       23              
  Lines        1530     1527       -3     
==========================================
+ Hits          966      972       +6     
+ Misses        474      465       -9     
  Partials       90       90              

Impacted Files	Coverage Δ
pkg/container/client.go	30.80% <50.00%> (+2.88%)	⬆️

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