Allow tarfiles to be built without --destination

[nessex]

Fixes #572 .

This uses the same hack as in build.go:576 of adding a dummy destination tag to the image, to ensure that a tarfile can be built without having to provide your own fake --destination flag.

I'm sure there's a neater way than this, but haven't wrapped my head around the codebase yet. Happy for a proper fix rather than this hack if you know a better way.

You can see the before / after results of this change in the debug container by using the following script:

#!/bin/sh

for t in `seq 1 3`
do
    mkdir -p "/test-${t}/out"

    cat <<-'EOF' > "/test-${t}/Dockerfile"
FROM scratch
COPY test.txt /test.txt
EOF

    cat <<-'EOF' > "/test-${t}/test.txt"
Hello, world!
EOF

done

set -x
# Test 1
executor -c /test-1 --tarPath /test-1/out/out.tar --verbosity=debug --no-push

# Test 2
executor -c /test-2 --tarPath /test-2/out/out.tar --verbosity=debug --destination=image

# Test 3
executor -c /test-3 --tarPath /test-3/out/out.tar --verbosity=debug --destination=image --no-push

# Output
tar -tvf /test-1/out/out.tar
tar -tvf /test-2/out/out.tar
tar -tvf /test-3/out/out.tar

Before this PR:

• Test 1 has only a manifest.json file with the contents null.

# Test 1: 
+ tar -tvf /test-1/out/out.tar
-rw-r--r-- 0/0         4 1970-01-01 00:00:00 manifest.json

# Test 2
+ tar -tvf /test-2/out/out.tar
tar: can't open '/test-2/out/out.tar': No such file or directory

# Test 3
+ tar -tvf /test-3/out/out.tar
-rw-r--r-- 0/0      1163 1970-01-01 00:00:00 sha256:b23ddc24a61ecdff8ed4bebb76e66c6b9745f3106500bbeee6adedfe9eba2191
-rw-r--r-- 0/0       164 1970-01-01 00:00:00 dd048939aae2fc6e215379ace348f79b10e5da6c6eeeb5bd28e6505fca027745.tar.gz
-rw-r--r-- 0/0       223 1970-01-01 00:00:00 manifest.json

After this PR:

# Test 1
+ tar -tvf /test-1/out/out.tar
-rw-r--r-- 0/0      1096 1970-01-01 00:00:00 sha256:8ddc1f7c7a1eda8ad87901febddcf1bac5a6c900690202f2fc1fb7a3c0c5feb1
-rw-r--r-- 0/0       157 1970-01-01 00:00:00 b572275845ad95d3c3cae56b385d055c9e0791b4799dfae218a3fdcc74e66700.tar.gz
-rw-r--r-- 0/0       218 1970-01-01 00:00:00 manifest.json

# Test 2
+ tar -tvf /test-2/out/out.tar
tar: can't open '/test-2/out/out.tar': No such file or directory

# Test 3
+ tar -tvf /test-3/out/out.tar
-rw-r--r-- 0/0      1096 1970-01-01 00:00:00 sha256:c73f5fd35c6a8e8e93d91bd4d0b18a9cda1fe4cff8c720665eeb9b1ce6e90244
-rw-r--r-- 0/0       157 1970-01-01 00:00:00 fbe8da3a8a5c5d3f2630e449baa556594e5d7e6ae48983f93649778d97c77717.tar.gz
-rw-r--r-- 0/0       257 1970-01-01 00:00:00 manifest.json

Expected Output:
Both Test 1 and Test 3 should generate a tarfile with valid layer contents, in this case a /test.txt file.

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[nessex]

I signed it!

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[tejal29]

Sorry for a late review, you shd check if there is no destination is provided.

[lnhrdt]

@nessex & @tejal29 is there any reason this PR didn't make it in? The README still suggests using the --destination=image hack but this PR seemed like a better resolution of the issue.

[lnhrdt]

To add some more clarity, the command succeeds without the --destination argument but produces an invalid image.tar with a file size of 0.

[nessex]

Apologies, I forgot this existed entirely, and never ended up using Kaniko due to this and other issues. Totally happy for someone to take over this change (code and all) and push it through, modify it or whatever.