Added exec command in sandbox

[yindia]

TL;DR

Added exec comamnd in sandbox

➜ flytectl sandbox exec -- ls -al
{total 72
drwxr-xr-x   24 root     root           768 Jun 25 17:33 .
drwxr-xr-x    1 root     root          4096 Jun 28 17:37 ..
-rw-r--r--    1 root     root           163 Jun  9 14:28 .flake8
drwxr-xr-x   16 root     root           512 Jun 28 08:18 .git
drwxr-xr-x    4 root     root           128 Jun  9 14:28 .github
-rw-r--r--    1 root     root           234 Jun 22 20:51 .gitignore
-rw-r--r--    1 root     root           318 Jun 25 16:10 .goreleaser.yml
drwxr-xr-x   11 root     root           352 Jun 28 15:15 .idea
-rw-r--r--    1 root     root           482 Jun  9 14:28 .readthedocs.yml
-rw-r--r--    1 root     root           143 Jun  9 14:28 CODEOWNERS
-rw-r--r--    1 root     root           158 Jun  9 14:28 CODE_OF_CONDUCT.md
-rw-r--r--    1 root     root         11341 Jun  9 14:28 LICENSE
-rw-r--r--    1 root     root          2933 Jun 25 16:10 Makefile
-rw-r--r--    1 root     root           184 Jun  9 14:28 NOTICE
-rw-r--r--    1 root     root          4240 Jun 25 16:10 README.md
drwxr-xr-x    5 root     root           160 Jun 22 15:20 boilerplate
drwxr-xr-x   26 root     root           832 Jun 25 16:10 cookbook
-rw-r--r--    1 root     root            44 Jun  9 14:28 katacoda.yaml
drwxr-xr-x    6 root     root           192 Jun  9 14:28 legacy
-rw-r--r--    1 root     root            20 Jun  9 14:28 prquantifier.yaml
-rw-r--r--    1 root     root            93 Jun 25 17:33 registries.yaml
drwxr-xr-x    2 root     root            64 Jun 23 05:55 release-snacks
drwxr-xr-x    3 root     root            96 Jun 12 08:34 tutorial
drwxr-xr-x    6 root     root           192 Jun 21 18:45 venv

Type

• Bug Fix
• Feature
• Plugin

Are all requirements met?

• Code completed
• Smoke tested
• Unit tests added
• Code documentation added
• Any pending items have an associated Issue

Complete description

How did you fix the bug, make the feature etc. Link to any design docs etc

Tracking Issue

https://github.com/lyft/flyte/issues/

Follow-up issue

NA
OR
https://github.com/lyft/flyte/issues/

[codecov]

Codecov Report

Merging #122 (1a98898) into master (e751a18) will increase coverage by 0.09%.
The diff coverage is 82.14%.

❗ Current head 1a98898 differs from pull request most recent head fc06edd. Consider uploading reports for the commit fc06edd to get more accurate results

@@            Coverage Diff             @@
##           master     #122      +/-   ##
==========================================
+ Coverage   73.54%   73.64%   +0.09%     
==========================================
  Files         104      105       +1     
  Lines        2491     2519      +28     
==========================================
+ Hits         1832     1855      +23     
- Misses        560      563       +3     
- Partials       99      101       +2     

Impacted Files	Coverage Δ
cmd/sandbox/exec.go	64.28% <64.28%> (ø)
cmd/sandbox/sandbox.go	100.00% <100.00%> (ø)
pkg/docker/docker_util.go	90.21% <100.00%> (ø)

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