Fix helm context path

Context

When working with the helm package registry, users can use helm push to publish their charts to GitLab.

To use helm push, users first must add GitLab as a repository. When added, the Helm client makes a request to download the chart index. This is used to help the Helm client know how to push helm charts with helm push.

The problem is, the returned chart index contains an incorrect value for contextPath. It returns a full URL rather than just a path, which leads the helm client to make a request to the wrong URL when using helm push.

What does this MR do?

Fix helm push, by setting the context path to a path (and not an URL).

See #18997 (comment 621773912) and !65760 (closed) for context.

Screenshots or Screencasts (strongly suggested)

Before (index request)

→ curl --user root:$TOKEN http://gdk.test:3001/api/v4/projects/83/packages/helm/stable/index.yaml
---
apiVersion: v1
entries:
  my-chart:
  - name: my-chart
    type: application
    version: 0.1.0
    apiVersion: v2
    appVersion: 1.16.0
    description: A Helm chart for Kubernetes
    created: '2021-07-07T22:22:58.351263000Z'
    digest:
    urls:
    - charts/my-chart-0.1.0.tgz
  test-gitlab-chart:
  - name: test-gitlab-chart
    type: application
    version: 0.1.0
    apiVersion: v2
    appVersion: 1.16.0
    description: A Helm chart for Kubernetes
    created: '2021-07-08T17:03:16.901701000Z'
    digest:
    urls:
    - charts/test-gitlab-chart-0.1.0.tgz
generated: '2021-07-08T17:17:45.892185000Z'
serverInfo:
  contextPath: "http://gdk.test:3001/api/v4/projects/83/packages/helm"

After (index request)

→ curl --user root:$TOKEN http://gdk.test:3001/api/v4/projects/83/packages/helm/stable/index.yaml
---
apiVersion: v1
entries:
  my-chart:
  - name: my-chart
    type: application
    version: 0.1.0
    apiVersion: v2
    appVersion: 1.16.0
    description: A Helm chart for Kubernetes
    created: '2021-07-07T22:22:58.351263000Z'
    digest:
    urls:
    - charts/my-chart-0.1.0.tgz
  test-gitlab-chart:
  - name: test-gitlab-chart
    type: application
    version: 0.1.0
    apiVersion: v2
    appVersion: 1.16.0
    description: A Helm chart for Kubernetes
    created: '2021-07-08T17:03:16.901701000Z'
    digest:
    urls:
    - charts/test-gitlab-chart-0.1.0.tgz
generated: '2021-07-08T17:17:45.892185000Z'
serverInfo:
  contextPath: "/api/v4/projects/83/packages/helm"

Before (helm push command)

# This results in an incorrect API request being made to http://gdk.test:3001/http:/gdk.test:3001/api/v4/projects/83/packages/helm/api/api/v4/projects/83/packages/helm/stable/charts
→ helm push test-gitlab-chart-0.1.0.tgz localproj
Pushing test-gitlab-chart-0.1.0.tgz to localproj...
Error: 404: could not properly parse response JSON: 

After (helm push command)

# This results in the correct API request being made to http://gdk.test:3001/api/v4/projects/83/packages/helm/api/stable/charts
→ helm push test-gitlab-chart-0.1.0.tgz localproj
Pushing test-gitlab-chart-0.1.0.tgz to localproj...
Done.

Does this MR meet the acceptance criteria?

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Does this MR contain changes to processing or storing of credentials or tokens, authorization and authentication methods or other items described in the security review guidelines? If not, then delete this Security section.

• Label as security and @ mention @gitlab-com/gl-security/appsec
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